feat(register): add cwd param

[aleclarson]

I'm hitting an edge case where my process.cwd() is not the value I want register to use.

This PR will let me circumvent that programatically.

require('tsconfig-paths').register({
  cwd: 'path/to/cwd',
})

[jonaskello]

Yes, this seems like a good idea :-) Could you update the README file to add this feature to the docs, and also the CHANGELOG file under the "Unreleased" heading?

[aleclarson]

Done ✅