Option for spell checking only lines that changed according to version control

[zroug]

It would be nice if there was an option to not check lines that didn't change according to version control.

Additionally there could be an option to add words that are already in version control to the dictionary. That would greatly reduce false positives because that way a project specific dictionary is created implicitly. Of course that does only work when spelling errors are reviewed before code is checked into version control, therefore this should be an off by default option.