Make ElixirFmt colors configurable and improve formatting

[Premwoik]

This PR addresses #25, #34, #30.

Changes

• Each atom is printed in double-quoted form : "dog" and then Elixir formatter decides when to get rid of quotes.
• Pretty printed types or expressions are now formatted by Elixir formatter.
• Add the possibility to change the configuration of the colors by passing it as an option.
• Now errors formatted in Gradient and Gradualizer use the same ASCII colors configuration.

[erszcz]

Whoa, this is awesome!