Split elixirDelimiter syntax group

`elixirDelimiter` group was splitted by functionality to get more flexibility for highlighting delimiters. So now there is `elixirStringDelimiter`, `elixirRegexDelimiter`, `elixirInterpolationDelimiter`, `elixirSigilDelimiter`
As an addition default highlighting for `elixirStringDelimiter` and `elixirInterpolationDelimiter` was changed

Before
![before](https://cloud.githubusercontent.com/assets/554231/4957210/5163e33c-66a2-11e4-926c-eb2a844a1e31.png)

After
![after](https://cloud.githubusercontent.com/assets/554231/4957214/55c819b6-66a2-11e4-8fa1-9b7db86599a7.png)


Could you change it back to `Delimiter`? I think it's better to show it as something different than the string.