Require format strings to use .format

[Lilja]

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First stab at #65. Defines a new root level matcher, pythonStringFormat. The name of this matcher I hope should resemble a string with an accompanying .format-function. Naming is a bit tricky since pythonFormat and pythonStrFormat are both taken.

This matcher tries to do a positive lookahead of .format if there are also strings begining with with ' or ".

No python 2 support. Personally I don't feel it's worth to add it, but if I can be persuaded to do so 😄

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[Lilja]

cc @nfnty

[nfnty]

I'd love for this functionality, but as it currently stands it's just not working correctly. If you want to tinker with this look for pythonString and try to implement a similar but custom syn-region but only for strings ending with .format(.