Allow REPL command aliasing

Fixes #730

You can now do:
`:alias "install" "i"`

In this case the new command is `:i` and you can use it as you would `:install`. Aliasing is implemented as another REPL command too.

This can obviously wait for the merge until the DEV branch accepts PRs again. 
This check can be removed if we put an extra condition into the `Where()` when setting `Commands`.