Add support for command aliases

* `session` and `window` commands are now know as `load-session` and
  `load-window` respectively, with aliases for their former names.
* Add short aliases for various commands.
* `help` and `completions` commands have been expanded with support for
  aliases.
* `help` command has been expanded upon with brief docs for a few more
  commands.

libexec/tmuxifier-edit-window

@@ -11,7 +11,7 @@ if [ "$1" == "--complete" ]; then
 fi
 
 if [ -z "$1" ]; then
-  echo "usage: tmuxifier edit-window <layout_name>" >&2
+  echo "$(tmuxifier-help edit-window)" >&2
   exit 1
 fi