Add additional option setting for clangtidy linter

The existing option setting handles setting additional compile flags to
pass to clang-tidy. The new option setting added here allows setting
additional clang-tidy specific flags to be passed as well.
Fixes #2324

doc/ale-c.txt

@@ -156,7 +156,7 @@ g:ale_c_clangtidy_options                           *g:ale_c_clangtidy_options*
   Type: |String|
   Default: `''`
 
-  This variable can be changed to modify flags given to clang-tidy.
+  This variable can be changed to modify compiler flags given to clang-tidy.
 
   - Setting this variable to a non-empty string,
   - and working in a buffer where no compilation database is found using
@@ -169,6 +169,14 @@ g:ale_c_clangtidy_options                           *g:ale_c_clangtidy_options*
   of the |g:ale_c_build_dir_names| directories of the project tree.
 
 
+g:ale_c_clangtidy_extra_options               *g:ale_c_clangtidy_extra_options*
+                                              *b:ale_c_clangtidy_extra_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to modify flags given to clang-tidy.
+
+
 ===============================================================================
 cppcheck                                                       *ale-c-cppcheck*