Add prettified symbols

[J3RN]

Emacs 24.4 (released circa 2014) introduced a minor mode called prettify-symbols-mode that, when enabled, will replace a string with a symbol according to the prettify-symbols-alist.

As such, and following loosely from rust-mode, I added a small set of pretty intuitive symbols for common Elixir constructs.

Before

After

[victorolinasc]

IMHO this should be at least optional and off-by-default. If we enable this by default I really think we are in for a big discussion thread about opinions.

We can, however, add this in a section in the README? I'm planning on a tips and tricks section to help newcomers and this could be in there.

Wdyt?

[victorolinasc]

I've added this configuration option in #505. Please, see if it does the trick for this optional feature.

If you still think we should add this built-in, please add one option so that this is opt-in (and not default). I've seen my share of font flame wars kkk. Even if default, we should provide an easy way to turn it off.

Thanks!