Scheme comparison

**Is your feature request related to a problem? Please describe.**
Compare Xcode project scheme's

**Describe the solution you'd like**
I would like to compare an Xcode project with a modified version, to see whether the schemes are identical or not.
It looks like the current version of `xcdiff` does not do this?

**Describe alternatives you've considered**
I have not considered any alternatives yet. I imagine I could just to a `diff`, but it would be nice if it was part of `xcdiff`

**Additional context**
n/a

Start by reviewing xcdiff's existing Xcode project diff entry point and how it handles scheme data. Determine how a modified project should be supplied and what scheme differences should be reported. Done means xcdiff can compare schemes between two projects and clearly identify whether they are identical.

Written by the indexing model from the issue text.