Converted snipmate elixir snippets into ultisnip

This adds support for Elixir snippets in Ultisnip by translating the existing elixir.snippets.

Credit belongs to @iurifq for the snippets
Translation from snipmate to ultisnip format @zph

Thanks for maintaining a good collection of snippets!

ZPH

Reference for confirmation to send PR by original snippets author: https://github.com/zph/ultisnips/commit/b9170d0333f54e51ed2551972c69bac40b6af96c#commitcomment-3782674