Make it clear that 'use_incremental' is a custom setup extension

[adiroiban]

When used in external projects, at first I was confused about the use_incremental argument for setup as I thought it was a standard setuptools argument,

maybe it can be renamed x_use_incremental or some other name to suggest that if people want to know more about that argument they should search in hawkowl/incremental and not in setuptools

[twm]

I don't think that this is worth doing as the use_foo idiom is incompatible with pyproject.toml. I've had to introduce a different approach to address #90.