Add support for cmake-format.

[antmusco]

Includes autoload/codefmt/cmakeformat.vim which adds support for cmake-format for formatting CMake files. Adds two additional flags, cmake_format_executable, which defaults to cmake-format, and cmake_format_config, which defaults to the empty string (i.e. default formatter).

Addresses issue https://github.com/google/vim-codefmt/issues/162

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[morxa]

@antmusco I'd really like to have support for cmake-format, so if you could pick this up again, I'd be very grateful :) Please let me know if I can help!

[dbarnett]

@morxa looks like this got abandoned. If you'd like to pick up the changes and send your own PR we can finish the review and try to get something merged.