Remove redundant semicolon to make tab() works better in ITerm 2.

I use that command a lot of times, and there's no shortcut for that.
Now it does :)

Some refactoring.

This prevents the previous behavior that exitted the zsh session
even if fbterm didn't start correctly.

Substitutes the current duplicate logic for the standard
and cross-platform function open_command in plugins:

  frontend-search, jira, node, web-search

Refactor the functions and group listings to have fewer functions,
but have them take group names as parameters.
Pull group definitions in to a single associative array.

Adding ability to add JIRA_PREFIX as an env variable (e.g. in .zshrc) and minor refactor to bash curly variable braces

virtualenvwrapper: treat git repos as project roots again (instead of requiring a .venv customization directory)
Change error output to more conventional OMZ format, so it's clear the plugin is for oh-my-zsh and not base zsh.
Use `local` variables instead of manual unsetting.