Move formatexpr config in-tree, out of plugin

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.

autoload/format.vim

@@ -0,0 +1,13 @@
+if !has('pythonx')
+  finish
+endif
+
+pythonx import format
+
+function! format#clang_format() abort
+  pythonx format.clang_format()
+endfunction
+
+function! format#yapf() abort
+  pythonx format.yapf()
+endfunction