Proposal: Strikethrough

[AriTheElk]

So I'm in the process of implementing a few features into markingbird and I wanted to run some implementations by you before opening up a pull request.

I've added strikethrough processing alongside bold and italics processing. Should I rename the current "boldItalic..." functions (like escapeBoldItalic()) to escapeBoldItalicStrike()? It seems unnecessary to add an additional function, when typically strikethrough should be processed at the same level as bold and italicized characters.

The other solution I see, is possibly moving to a more generic naming scheme, so that the function names wouldn't have to change with new additions. But perhaps its better to limit this function group to just bold, italic, and strikethrough.

[kristopherjohnson]

As strikethrough is not part of John Gruber's original Markdown spec (see http://daringfireball.net/linked/2015/11/05/markdown-strikethrough-slack), I would prefer that it be added as a separate function, and that it be enableable/disableable via MarkdownOptions.

[AriTheElk]

Ah I see. In that case maybe I'm better off making a github flavored fork of this.