Add configurable build message

Having a hard-coded HTML description is pretty inconvenient when you don't want to allow raw HTML descriptions, or if you'd rather have an altogether different build description message. This pull request adds a customisable template for the build message.
To answer @valdisrigdon, if you ever delete the build description and save the config then it will be lost, and the only way to get it back would be to recreate it manually.  @pdpi your value was set, I am assuming, because it was the first time installing and running the plugin, so it was saved to the xml file, then once you delete the field it would remain blank.