Introduces the `format` Python module which provides `clang_format()`
and `yapf()` functions which efficiently (compared to vimscript) invoke
`clang-format` or `yapf` respectively then apply the minimal number of
changes using `difflib.SequenceMatcher`.
Additionally, in order to invoke these Python functions add |autoload|
functions `format#clang_format()` and `format#yapf()` which can be
directly used by the 'formatexpr' setting.
Finally, add |ftplugin| files which set 'formatexpr' when the |autoload|
functions are available.
