Make BUILD_REQUEST_MARKER configurable

Quick change to add an input field to the plugin configuration to set the comment phrase the plugin looks for to trigger a rebuild. My team wasn't too happy with using "test this please" as it was ambiguous as to whom the message was directed.

Note: I really don't know java at all. This built and ran for me, but please read the diff carefully. Thanks!
Agreed..I've updated my PR, sorry for the delay!