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-load-window

@@ -0,0 +1,22 @@
+#! /usr/bin/env bash
+set -e
+[ -n "$TMUXIFIER_DEBUG" ] && set -x
+
+# Provide tmuxifier completions
+if [ "$1" == "--complete" ]; then
+  for item in $(tmuxifier-list-windows); do
+    echo "$item"
+  done
+  exit
+fi
+
+# Load runtime functions.
+source "$TMUXIFIER/runtime.sh"
+
+if [ ! -z $TMUX ]; then
+  session="$(tmuxifier-current-session)"
+  load_window "$1"
+else
+  echo "tmuxifier: 'window' command can only be used from within Tmux."
+  exit 1
+fi