Regex aliases

Hi! This PR concerns the `:FanfingTasticAlias` command. It seems to have been broken, so I fixed it. Then I allowed it to take regex, inspired by [tpope/vim-flatfoot](https://github.com/tpope/vim-flatfoot). I added some documentation too. I don't know if it was originally intended to be supported, but the docs mention that mapping control characters like `<C-D>` is allowed.