Add support for jira-prefix configuration file

This adds the option of an extra configuration file for specifying a prefix for JIRA issues. Matching the pattern of the existing plugin, this configuration file can be system-wide or project specific.

This would allow you to just type the numeric ID of an issue rather than including the project prefix. E.g. "jira 123" rather than "jira CHROME-123".

Note that this will conflict with PR2898 if that was merged first, but resolving on either side would be trivial.