Allow generic self-closing blocks

[StephenHogsten]

Right not it looks like all the possible elements that can be self-closed without adding an indentation level are hard-coded. Which means if you have some sort of custom element (e.g. later replaced by vue) that is self-closed, it will break the indentation formatting

[threedaymonk]

This probably needs to be available as a command line option with the ability to load defaults from a configuration file. I'd be open to a PR!