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-completions

@@ -2,14 +2,27 @@
 set -e
 [ -n "$TMUXIFIER_DEBUG" ] && set -x
 
+has-completions() {
+  grep -i "^# provide tmuxifier completions" "$1" >/dev/null
+}
+
 command="$1"
 if [ -z "$command" ]; then
   echo "Usage: tmuxifier completions COMMAND [arg1 arg2...]" >&2
   exit 1
 fi
 
-command_path="$(command -v "tmuxifier-$command")"
-if grep -i "^# provide tmuxifier completions" "$command_path" >/dev/null; then
+command_path="$(command -v "tmuxifier-$command" || true)"
+
+# Attempt to resolve aliases
+if [ -z "$command_path" ]; then
+  resolved="$(tmuxifier-alias "$command")"
+  if [ ! -z "$resolved" ]; then
+    command_path="$(command -v "tmuxifier-$resolved" || true)"
+  fi
+fi
+
+if [ ! -z "$command_path" ] && has-completions "$command_path"; then
   shift
   exec "$command_path" --complete "$@"
 fi