Add a linter for alex

https://github.com/wooorm/alex

Enabled for text-like file formats and documented in README and doc.

ale_linters/xhtml/alex.vim

@@ -0,0 +1,10 @@
+" Author: Johannes Wienke <languitar@semipol.de>
+" Description: alex for XHTML files
+
+call ale#linter#Define('xhtml', {
+\   'name': 'alex',
+\   'executable': 'alex',
+\   'command': 'alex %t -t',
+\   'output_stream': 'stderr',
+\   'callback': 'ale#handlers#alex#Handle',
+\})