Add feature for `-/+` instead of `</>`

[spenserblack]

Fixes #35

If users prefer -/+ to </>, they can add this to Cargo.toml:

[dev-dependencies]
pretty_assertions = { version = "*", features = ["git-style-diff"] }

This probably isn't the best name for the feature, so I'm open to suggestions :laughing:

[rivy]

I included the same in the "now configurable" patch that I've posted as a WIP (#38). I called it "diffstyle_git" ... though I'm not super-happy about that either. 😄

[spenserblack]

@colin-kiegel Any chance of a review of this PR or #38?