Break shared handlers up into their own files, and fix stylelint error handling

ale_linters/cmake/cmakelint.vim

@@ -20,5 +20,5 @@ call ale#linter#Define('cmake', {
 \   'name': 'cmakelint',
 \   'executable_callback': 'ale_linters#cmake#cmakelint#Executable',
 \   'command_callback': 'ale_linters#cmake#cmakelint#Command',
-\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
 \})