Add :ToGithubBlame to open blame route for same file location

[synthead]

Closes https://github.com/tonchis/vim-to-github/issues/14!

This PR adds :ToGithubBlame so users can open the "blame" route in GitHub for the same file location.

This is achieved simply by replacing "blob" with "blame" in the URL. For example, for this "blob" route...

https://github.com/tonchis/vim-to-github/blob/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/plugin/to-github.vim#L51-L59

...we can also use this "blame" route:

https://github.com/tonchis/vim-to-github/blame/9ea9c75b6cd48bd42823a39c56a05a2ff8161536/plugin/to-github.vim#L51-L59