Rename options structures

Remove the use of `_opts` in structure names, as mentioned in #2014.

- [x] `git_checkout_opts`
- [x] `git_revert_opts`
- [x] Update the documentation site (https://github.com/libgit2/libgit2.github.com/pull/44)
- [x] ~~Eliminate `git_merge_opts`~~
- [x] ~~`git_merge_tree_opts` :point_right: `git_merge_options`~~

(Those last two are speculative, depending on how @ethomson feels.)