Highlight %-sigils as builtins

This helps make %B and friends stand out a little more.
Highlight @ expressions and make them a prefix char

The prefix-char-ness lets these expressions be treated as a single
token by smie.
Add some indentation regression tests.

* New command `run-elixir-tests' for running tests on elixir snippets
  in temporary buffers
* New file elixir-tests.el that gets (re-)loaded by that command,
  containing a few of these snippets.