Support themes that extend material in reconfigure

[nejch]

The plugin's reconfigure functionality seems to be a little too strict on detecting mkdocs-material, so themes that extend it cannot make use of the link awareness for the switcher it seems. See:

mkdocs-static-i18n/mkdocs_static_i18n/reconfigure.py

Lines 126 to 128 in 1d1501d

	# material theme specific reconfiguration (can be disabled)
	if config.theme.name == "material" and self.config.reconfigure_material is True:
	# check and warn about missing mkdocs-material version

I think something like an allowed_themes plugin option defaulting to material might be enough, and people can override it then if needed. WDYT?

-        if config.theme.name == "material" and self.config.reconfigure_material is True:
+        if config.theme.name in self.config.allowed_themes and self.config.reconfigure_material is True: 

(or simply drop the if config.theme.name == "material" requirement and just look for the installed package)