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

ale_linters/sass/sasslint.vim

@@ -4,5 +4,5 @@ call ale#linter#Define('sass', {
 \   'name': 'sasslint',
 \   'executable': 'sass-lint',
 \   'command': 'sass-lint -v -q -f compact %t',
-\   'callback': 'ale#handlers#HandleCSSLintFormat',
+\   'callback': 'ale#handlers#css#HandleCSSLintFormat',
 \})

ale_linters/sass/stylelint.vim

@@ -27,5 +27,5 @@ call ale#linter#Define('sass', {
 \   'name': 'stylelint',
 \   'executable_callback': 'ale_linters#sass#stylelint#GetExecutable',
 \   'command_callback': 'ale_linters#sass#stylelint#GetCommand',
-\   'callback': 'ale#handlers#HandleStyleLintFormat',
+\   'callback': 'ale#handlers#css#HandleStyleLintFormat',
 \})