Add optional line highlightning

Adds 3 public functions to enable/disable/toggle line
highlightning of added/modified/removed lines using the
DiffAdd, DiffChange and DiffDelete highlight groups respectively.
This can be used to give a more 'obstrusive' feedback of changes
in the buffer.

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The commit message says it all. While this isn't an omg-amazing kind of feature i find it kind of useful as a more obvious feedback when i quickly want to skim the buffer for changes that i've done before committing. Also it's only a few lines of code anyway.