Add flag to disable implicit markdown formatting for non-doc line comments

[andrewbaxter]

Fix #103

This adds the explicit_markdown_comments config option which disables implicit (normal) comment markdown formatting as well as the //? comment that's an explicitly markdown formatted comment (parallel to //. for an explicitly non-markdown formatted comment).

The above was added to the readme too.

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I agree that when the request is merged I assign the copyright of the request to the repository owner.

[andrewbaxter]

@rootnas @AaronKutch I think this should solve the "don't format // part". I guess given the amount of traffic this repo gets and the size of this change I may go ahead and merge and hope if there are issues they aren't disrupting too many people...

[andrewbaxter]

Released in genemichaels 0.6.0