patterns.py: add github_username pattern

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

src/patchtest2/patterns.py

@@ -56,6 +56,8 @@
 mbox_bugzilla_validation = pyparsing.Regex("\[(\s?YOCTO\s?#\s?(\d+)\s?,?)+\]")
 mbox_revert_shortlog_regex = pyparsing.Regex('Revert\s+".*"')
 mbox_shortlog_maxlength = 90
+# based on https://stackoverflow.com/questions/30281026/regex-parsing-github-usernames-javascript
+mbox_github_username = pyparsing.Regex('\B@([a-z0-9](?:-(?=[a-z0-9])|[a-z0-9]){0,38}(?<=[a-z0-9]))')
 
 # patch