Add support for multiple tags in a single FrameConverter

[Cadair]

Don't assume that tag is always a string.

[Cadair]

To add some detail, what I am suggesting is making it so:

asdf-astropy/asdf_astropy/converters/coordinates/frame.py

Lines 12 to 14 in 5d3b444

	@property
	def tags(self):
	return [self._tag]

this becomes return self._tags so you can specify more than one tag per converter.