Allow clangformat to use a local style file (#3587)

* Allow clangformat to use a local style file.

* Add tests.

* Fix Vint issue.

* Improve explanation of feature in documentation.

* Fix failing test.

The test was checking the wrong directory.

doc/ale-c.txt

@@ -202,7 +202,45 @@ g:ale_c_clangformat_options                       *g:ale_c_clangformat_options*
   Type: |String|
   Default: `''`
 
-  This variable can be change to modify flags given to clang-format.
+  This variable can be changed to modify flags given to clang-format.
+
+
+g:ale_c_clangformat_style_option             *g:ale_c_clangformat_style_option*
+                                             *b:ale_c_clangformat_style_option*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to modify only the style flag given to
+  clang-format. The contents of the variable are passed directly to the -style
+  flag of clang-format.
+
+  Example: >
+      {
+    \ BasedOnStyle:                        Microsoft,
+    \ ColumnLimit:                         80,
+    \ AllowShortBlocksOnASingleLine:       Always,
+    \ AllowShortFunctionsOnASingleLine:    Inline,
+    \ }
+<
+  If you set this variable, ensure you don't modify -style in
+  |g:ale_c_clangformat_options|, as this will cause clang-format to error.
+
+
+g:ale_c_clangformat_use_local_file         *g:ale_c_clangformat_use_local_file*
+                                           *b:ale_c_clangformat_use_local_file*
+  Type: |Number|
+  Default: `0`
+
+  This variable can be changed to modify whether to use a local .clang-format
+  file. If the file is found, the flag '-style=file' is passed to clang-format
+  and any options configured via |g:ale_c_clangformat_style_option| are not
+  passed.
+
+  If this option is enabled but no .clang-format file is found, default back to
+  |g:ale_c_clangformat_style_option|, if it set.
+
+  If you set this variable, ensure you don't modify -style in
+  |g:ale_c_clangformat_options|, as this will cause clang-format to error.
 
 
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