HTML attribution about xyzservices

[darribas]

This is a good idea. If nothing else is available (I think some providers do suggest HTML), we could have a build_html_attribution method with a simple yet stylish version?

[jorisvandenbossche]

For the leaflet provider sources, the original actually has html attribution, which currently gets converted to plain text (for simple usage with matplotlib):

xyzservices/provider_sources/_parse_leaflet_providers.py

Lines 123 to 126 in ee5ade0

	# convert html text to plain text
	converter = html2text.HTML2Text(bodywidth=1000)
	converter.ignore_links = True
	value = converter.handle(value).strip()

So for those sources it should be easy to keep both in the parsed data.