Sigil support

* Support for [heredoc sigils using `"""` and `'''`](https://github.com/elixir-lang/elixir/blob/e00db531122641907fa201ed6f45daf0d48caa8a/lib/elixir/src/elixir_tokenizer.erl#L141-L148).
* Support for [matched terminator sigils](https://github.com/elixir-lang/elixir/blob/e00db531122641907fa201ed6f45daf0d48caa8a/lib/elixir/src/elixir_tokenizer.erl#L150-L158)
  * `/`
  * `<` and `>`
  * `"`
  * `'`
  * `[` and `]`
  * `(` and `)`
  * `{` and `}`
  * `|`