Add comma as valid character in synopsis parser

Add a comma as a valid character in the value field of the synopsis parser.
Example:
[--allowed_themes=<themeA,themeB>]
set only the MYSQL_PWD environment variable

this avoids having to use the unreliable $_ENV superglobal

fixes #950
Merge pull request #1013 from sboisvert/master

Add comma as valid character in synopsis parser