Add SyntaxErl linter

These changes add [SyntaxErl][1] integration.  SyntaxErl is a syntax
checker tool for Erlang.

[1]: https://github.com/ten0s/syntaxerl

test/handler/test_syntaxerl_handler.vader

@@ -0,0 +1,24 @@
+Before:
+  runtime ale_linters/erlang/syntaxerl.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute:
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 42,
+  \     'text': "syntax error before: ','",
+  \     'type': 'E',
+  \   },
+  \   {
+  \     'lnum': 42,
+  \     'text': 'function foo/0 is unused',
+  \     'type': 'W',
+  \   },
+  \ ],
+  \ ale_linters#erlang#syntaxerl#Handle(42, [
+  \   "/tmp/v2wDixk/1/module.erl:42: syntax error before: ','",
+  \   '/tmp/v2wDixk/2/module.erl:42: warning: function foo/0 is unused',
+  \ ])