Resolves #92

path.relative(from, to) is weird when you have an absolute path and a relative path. By normalizing p to path.join(cwd, p) to get an absolute path it works.

I added some additional guards here just to be really safe about it:

• If p is already an absolute path:
  • Assert that it is in cwd and return it
• Otherwise, path.join(cwd, p)

I don't know if these cases will actually come up, but it seemed better to give a specific error message and guard it a bit more.