Updates to regexes for various lexemes

This patch switches `elixir-mode` to use the lexemes/tokens outlined [here](https://github.com/alco/ace-mode-elixir/blob/7a841ee05ac3b5d676ddc3f1397713765a9f87fa/mode-elixir.js#L244-L269) by @alco. Their list of lexemes is up-to-date with Elixir master, whereas ours was of questionable heritage.

This patch also adds support for highlighting keyword-style maps (e.g.

```elixir
%{a: 1, b: 2}
```