Guillemet support

[fiapps]

Several languages, including French and Italian, use guillemets «» to mark quotations. Wikipedia says they are also known as angle quotes. I would like MacDown to support them in the same way that American English quotation marks are supported.

Currently, typing " produces a pair of quotation marks (""), and if text is selected, one is placed before the selected text and another after (hello -> "hello"). So if I type «, I would like «» to be produced, and if text is selected when I type «, I would like the selected text to be wrapped in guillemets (hello -> «hello»).