Support for custom tags

[gdude2002]

In the same project mentioned in #91, we have a number of custom Pebble tags available for writers to use. For example:

These tags are not part of this project - they're part of a separate project that we wrote to build the files in the first project.

I'd like to see a way to tell the plugin that the custom tags exist in pairs - currently, the plugin is unable to match the start tag to the end tag, and thus auto-formatting the file does, well, this.

Note that some tags contain multiple words separated with underscores, so the plugin would need to be able to handle end_ as a prefix, and not just end.