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

@@ -2,7 +2,6 @@ if !has('pythonx')
   finish
 endif
 
-" set debug=msg,throw
 pythonx import format
 
 function! format#clang_format() abort

ftplugin/c.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/cpp.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/java.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/javascript.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/objc.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/objcpp.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/proto.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#clang_format')
   set formatexpr=format#clang_format()
 endif

ftplugin/python.vim

@@ -1,3 +1,3 @@
-if has('pythonx')
+if exists('*format#yapf')
   set formatexpr=format#yapf()
 endif