feat(index): add literate: 'auto', make it default

Add new literate option value - 'auto':
- treat .litcoffee and .coffee.md files as literate CoffeeScript
- assume everything else is CoffeeScript

This change simplifies gulp setup for projects with mixed .coffee
and .litcoffee files.

Add tests for it in test/literate-detection.coffee.
Update readme.md accordingly.

BREAKING CHANGE: literate default value changes from false to 'auto'.
If you somewhy lint .litcoffee or .coffee.md files as CoffeeScript
this may change linting results.
To fix this explicitly set literate to false.