standardize plugins, update metadata

add about-plugin metadata
chmod -x plugins
cleanup filenames to standardize on x.plugin.bash format
only plugin files intended to be executable from the command line should
contain a shebang line, and should be a+x.

plugins/available/rbenv.plugin.bash

@@ -1,8 +1,10 @@
-#!/usr/bin/env bash
-
 # Load rbebv, if you are using it
+
+cite about-plugin
+about-plugin 'load rbenv, if you are using it'
+
 export PATH="$HOME/.rbenv/bin:$PATH"
 [[ `which rbenv` ]] && eval "$(rbenv init -)"
 
 # Load the auto-completion script if rbenv was loaded.
-[[ -e ~/.rbenv/completions/rbenv.bash ]] && source ~/.rbenv/completions/rbenv.bash
+[[ -e ~/.rbenv/completions/rbenv.bash ]] && source ~/.rbenv/completions/rbenv.bash