Allow passing merge options to merge

[rohitsanj]

This PR reimplements the merge method using cffi and removes the C implementation.

In addition, the merge method will now accept additional arguments favor, flags and file_flags to have more control over merge options.

The existing merge tests pass. I'm not sure what other cases need to be tested.

[jdavid]

Can you rebase and squash?
(When syncing changes from master prefer rebase over merge)

[jdavid]

Looks good, do you think you could add unit tests for the options?
There are similar unit tests for other parts of the API, e.g. test_merge_commits_favor

[rohitsanj]

Sure, I can add unit tests.

I was confident about the functionality of the merge options passthrough in this implementation since the _merge_options method seems decently unit tested.