feat(elixir): add support for elixir bitstrings

[c-alpha]

Elixir bitsrings are something I use often, but have been sorely absent from smartparens.

To complete the present PR, I would need a bit of a helping hand with creating test cases though.

Bitsrings are enclosed in << and >>, and in parsing can be mistaken for the operators <<~ and <<<. Thus, << followed by anything but ~ or < starts a bitsring.

Examples:

• <<tag::5, length::2, mandatory::1>> = data parses the first eight bits in data
• the bitsring can also be padded with whitespace and/or split across lines:

<<
  tag::5,
  length::2,
  mandatory::1
>> = data

• not bitsrings are a <<< b and a <<~ b

Where can I find examples and documentation for creating test cases?