New linter: dls (#1992)

* New linter: dls

test/command_callback/test_d_dls_callbacks.vader

@@ -0,0 +1,19 @@
+Before:
+  call ale#assert#SetUpLinterTest('d', 'dls')
+
+  Save &filetype
+  let &filetype = 'd'
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The language string should be correct):
+  AssertLSPLanguage 'd'
+
+Execute(The default executable should be correct):
+  AssertLinter 'dls', 'dls'
+
+Execute(The executable should be configurable):
+  let g:ale_d_dls_executable = 'foobar'
+
+  AssertLinter 'foobar', 'foobar'