Add support for Swift with swiftlint (#214)

* Add support for swift with swiftlint

* Fix issue by adding '.swift' file extension

ale_linters/swift/swiftlint.vim

@@ -0,0 +1,9 @@
+" Author: David Mohundro <david@mohundro.com>
+" Description: swiftlint for swift files
+
+call ale#linter#Define('swiftlint', {
+\   'name': 'swiftlint',
+\   'executable': 'swiftlint',
+\   'command': g:ale#util#stdin_wrapper . ' .swift swiftlint',
+\   'callback': 'ale#handlers#HandleGCCFormat',
+\})