feat(plugins): Add Ag completion (#3534)

* Add Ag completion

A replacement for ack/grep https://github.com/ggreer/the_silver_searcher

* Create README.md

As requested in the PR: https://github.com/ohmyzsh/ohmyzsh/pull/3534

plugins/ag/README.md

+# The Silver Searcher
+
+This plugin provides completion support for [`ag`](https://github.com/ggreer/the_silver_searcher).
+
+To use it, add ag to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... aws)
+```
+
+## INSTALLATION NOTES
+
+Besides oh-my-zsh, `ag` needs to be installed by following these steps: https://github.com/ggreer/the_silver_searcher#installing.

plugins/ag/_ag

+#compdef ag
+#autoload
+
+typeset -A opt_args
+
+# Took the liberty of not listing every option… specially aliases and -D
+_ag () {
+  local -a _1st_arguments
+  _1st_arguments=(
+    '--ackmate:Print results in AckMate-parseable format'
+    {'-A','--after'}':[LINES] Print lines after match (Default: 2)'
+    {'-B','--before'}':[LINES] Print lines before match (Default: 2)'
+    '--break:Print newlines between matches in different files'
+    '--nobreak:Do not print newlines between matches in different files'
+    {'-c','--count'}':Only print the number of matches in each file'
+    '--color:Print color codes in results (Default: On)'
+    '--nocolor:Do not print color codes in results'
+    '--color-line-number:Color codes for line numbers (Default: 1;33)'
+    '--color-match:Color codes for result match numbers (Default: 30;43)'
+    '--color-path:Color codes for path names (Default: 1;32)'
+    '--column:Print column numbers in results'
+    {'-H','--heading'}':Print file names (On unless searching a single file)'
+    '--noheading:Do not print file names (On unless searching a single file)'
+    '--line-numbers:Print line numbers even for streams'
+    {'-C','--context'}':[LINES] Print lines before and after matches (Default: 2)'
+    '-g:[PATTERN] Print filenames matching PATTERN'
+    {'-l','--files-with-matches'}':Only print filenames that contain matches'
+    {'-L','--files-without-matches'}':Only print filenames that do not contain matches'
+    '--no-numbers:Do not print line numbers'
+    {'-o','--only-matching'}':Prints only the matching part of the lines'
+    '--print-long-lines:Print matches on very long lines (Default: 2k characters)'
+    '--passthrough:When searching a stream, print all lines even if they do not match'
+    '--silent:Suppress all log messages, including errors'
+    '--stats:Print stats (files scanned, time taken, etc.)'
+    '--vimgrep:Print results like vim :vimgrep /pattern/g would'
+    {'-0','--null'}':Separate filenames with null (for "xargs -0")'
+
+    {'-a','--all-types'}':Search all files (does not include hidden files / .gitignore)'
+    '--depth:[NUM] Search up to NUM directories deep (Default: 25)'
+    {'-f','--follow'}':Follow symlinks'
+    {'-G','--file-search-regex'}':[PATTERN] Limit search to filenames matching PATTERN'
+    '--hidden:Search hidden files (obeys .*ignore files)'
+    {'-i','--ignore-case'}':Match case insensitively'
+    '--ignore:[PATTERN] Ignore files/directories matching PATTERN'
+    {'-m','--max-count'}':[NUM] Skip the rest of a file after NUM matches (Default: 10k)'
+    {'-p','--path-to-agignore'}':[PATH] Use .agignore file at PATH'
+    {'-Q','--literal'}':Do not parse PATTERN as a regular expression'
+    {'-s','--case-sensitive'}':Match case'
+    {'-S','--smart-case'}':Insensitive match unless PATTERN has uppercase (Default: On)'
+    '--search-binary:Search binary files for matches'
+    {'-t','--all-text'}':Search all text files (Hidden files not included)'
+    {'-u','--unrestricted'}':Search all files (ignore .agignore and _all_)'
+    {'-U','--skip-vcs-ignores'}':Ignore VCS files (stil obey .agignore)'
+    {'-v','--invert-match'}':Invert match'
+    {'-w','--word-regexp'}':Only match whole words'
+    {'-z','--search-zip'}':Search contents of compressed (e.g., gzip) files'
+
+    '--list-file-types:list of supported file types'
+  )
+
+  if [[ $words[-1] =~ "^-" ]]; then
+    _describe -t commands "ag options" _1st_arguments && ret=0
+  else
+    _files && ret=0
+  fi
+}