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. 
`&& replCommands.Any(x => x.CommandName != null)` is made redundant by the `Where()`