Add SyntaxErl linter

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

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

ale_linters/erlang/syntaxerl.vim

@@ -0,0 +1,38 @@
+" Author: Dmitri Vereshchagin <dmitri.vereshchagin@gmail.com>
+" Description: SyntaxErl linter for Erlang files
+
+call ale#Set('erlang_syntaxerl_executable', 'syntaxerl')
+
+
+function! ale_linters#erlang#syntaxerl#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'erlang_syntaxerl_executable')
+endfunction
+
+
+function! ale_linters#erlang#syntaxerl#GetCommand(buffer) abort
+    return ale_linters#erlang#syntaxerl#GetExecutable(a:buffer) . ' %t'
+endfunction
+
+
+function! ale_linters#erlang#syntaxerl#Handle(buffer, lines) abort
+    let l:pattern = '\v\C:(\d+):( warning:)? (.+)'
+    let l:loclist = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:pattern)
+        call add(l:loclist, {
+        \   'lnum': l:match[1] + 0,
+        \   'text': l:match[3],
+        \   'type': empty(l:match[2]) ? 'E' : 'W',
+        \})
+    endfor
+
+    return l:loclist
+endfunction
+
+
+call ale#linter#Define('erlang', {
+\   'name': 'syntaxerl',
+\   'executable_callback': 'ale_linters#erlang#syntaxerl#GetExecutable',
+\   'command_callback': 'ale_linters#erlang#syntaxerl#GetCommand',
+\   'callback': 'ale_linters#erlang#syntaxerl#Handle',
+\})