Added options to retain original typehints in signatures

[BhavyeMathur]

I was using this library and noticed that there was no existing way to retain the typehints automatically generated by autodoc in function/class/method signatures. I quite liked the way that return types are annotated with the -> in signatures and wanted to preserve those while using autodoc-typehints.

This pull request adds 2 simple options that do not alter the behaviour of anyone already using typehints:

1. typehints_use_signature (False by default)
2. typehints_use_signature_return (False by default)

By setting these, the sph_signature.replace() commands in process_signature() can be altered

With default options:

With the new options set to True:

[gaborbernat]

Tests, please!

[BhavyeMathur]

Hey! So I'm new to pull requests and to sphinx. Any particular tests you're looking for?

[gaborbernat]

Test for all features you've added.