Add option to ignore whitespace changes

[RandScullard]

I would love to be able to toggle whitespace sensitivity on and off in GitDiffMargin. (Equivalent to passing the --ignore-space-change option to git diff.)

When I have changed the indentation of a large block of code, I usually don't want to see the entire block highlighted, just the lines I've materially changed.

[PavelA85]

+1

[ismusidhu]

+1

[glconti]

This is addressed in #116

[HamRusTal]

As an alternative to a plain on/off, I'd prefer to be able to still see the whitespace-only changes, yet with another margin color (paler, configurable).