Add XO as a linter/fixer for TypeScript files (#2453)

test/command_callback/test_xo_command_callback.vader

@@ -0,0 +1,20 @@
+Before:
+  call ale#assert#SetUpLinterTest('typescript', 'xo')
+  call ale#test#SetFilename('testfile.ts')
+  unlet! b:executable
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The XO executable should be called):
+  AssertLinter 'xo', ale#Escape('xo') . ' --reporter unix --stdin --stdin-filename %s'
+
+Execute(The XO executable should be configurable):
+  let b:ale_typescript_xo_executable = 'foobar'
+
+  AssertLinter 'foobar', ale#Escape('foobar') . ' --reporter unix --stdin --stdin-filename %s'
+
+Execute(The XO options should be configurable):
+  let b:ale_typescript_xo_options = '--wat'
+
+  AssertLinter 'xo', ale#Escape('xo') . ' --wat --reporter unix --stdin --stdin-filename %s'