Fix passing atom to mix task :main option

Looks like this code wasn't kept in sync with the Elixir main repo.
Merge pull request #82 from pminten/fix-options-main-atom-update

Fix passing atom to mix task :main option
Add lazy mix.exs options
Allow nice codeblocks for README

This is an ugly hack, but that's because our markdown pipeline isn't that great.
Merge pull request #84 from pminten/codeblocks-for-readme

Allow nice codeblocks for README
Merge pull request #83 from ericmj/mix-lazy-options

Add lazy mix.exs options
Add support for fenced code
Fix build on master
Add tests to Mix.Tasks.Docs
Merge pull request #85 from ericmj/fenced-code

Add support for fenced code
Get rid of ANSI code
Generate markdown.so into priv directory
Provide default docs for callback implementations

This only triggers if there is no doc present and the module
has correctly declared @behaviour (in the correct spelling).
Merge pull request #88 from pminten/callback-implementations

Provide default docs for callback implementations