Only enable swiftlint checker with config file

This means that if you just have swiftlint installed, it won't be
enabled automatically. You either need a `.swiftlint.yml` or you need to
set `g:syntastic_swift_swiftlint_use_defaults` to 1 in your vimrc.

syntax_checkers/swift/swiftlint.vim

@@ -6,6 +6,15 @@ let g:loaded_syntastic_swift_swiftlint_checker = 1
 let s:save_cpo = &cpo
 set cpo&vim
 
+function! SyntaxCheckers_swift_swiftlint_IsAvailable() dict
+  if !executable(self.getExec())
+    return 0
+  endif
+
+  return get(g:, 'syntastic_swift_swiftlint_use_defaults', 0)
+        \ || filereadable('.swiftlint.yml')
+endfunction
+
 function! SyntaxCheckers_swift_swiftlint_GetLocList() dict
   let env_vars = 'SCRIPT_INPUT_FILE_COUNT=1 SCRIPT_INPUT_FILE_0=' . syntastic#util#shexpand('%:p')
   let makeprg = self.makeprgBuild({