provide plugin for rbfu the ruby version switcher

Evaluates $(rbfu --init --auto) if rbfu is installed on the system,
also has a custom rbfu-rubies method which lists the installed
rubies which are available to rbfu.

For compatibility with themes, also creates the rvm_prompt_info
function, so existing themes should work out of the box.

plugins/rbfu/rbfu.plugin.zsh

+# Enables rbfu with --auto option, if available.
+#
+# Also provides a command to list all installed/available
+# rubies. To ensure compatibility with themes, creates the
+# rvm_prompt_info function to return the $RBFU_RUBY_VERSION
+# version.
+
+command -v rbfu &>/dev/null
+
+if [[ $? -eq 0 ]]; then
+  eval "$(rbfu --init --auto)"
+
+  # Internal: Print ruby version details, if it's currently
+  # active etc.
+  function _rbfu_rubies_print() {
+    local rb rb_out
+    rb=$(basename $1)
+    rb_out="$rb"
+    [[ -h $1 ]] && rb_out="$rb_out${fg[green]}@${reset_color}"
+    [[ "x$rb" == "x$2" ]] && rb_out="${fg[red]}$rb_out ${fg[red]}*${reset_color}"
+    echo $rb_out
+  }
+
+  # Public: Provide a list with all available rubies, this basically depends
+  # on `ls -1` and .rfbu/rubies. Highlights the currently active ruby version
+  # and aliases.
+  function rbfu-rubies() {
+    local rbfu_dir active_rb
+    rbfu_dir=$RBFU_RUBIES
+    active_rb=$RBFU_RUBY_VERSION
+    [[ -z "$rbfu_dir" ]] && rbfu_dir="${HOME}/.rbfu/rubies"
+    [[ -z "$active_rb" ]] && active_rb="system"
+    _rbfu_rubies_print "${rbfu_dir}/system" $active_rb
+    for rb in $(ls -1 $rbfu_dir); do
+      _rbfu_rubies_print "${rbfu_dir}/${rb}" $active_rb
+    done
+  }
+
+  # Public: Create rvm_prompt_info command for themes compatibility, unless
+  # it has already been defined.
+  [ ! -x rvm_prompt_info ] && function rvm_prompt_info() { echo "${RBFU_RUBY_VERSION:=system}" }
+fi