Add callback support for handling unknown tags

[subsoap]

Allow the user to setup a callback function to run if the user wants to use tags which are not defined in the normal extension.

Example for a possible action tag would be

<action=particles:text_emitter>Make this pretty</action>

and

<action=sound:ding/>

The main thing the callback needs to be used for is defining how the data of the tag is handled so in the sound case the "sound:ding" would need to be handled when doing parse_tag.

local parse = require("richtext.parse")
parse.callback = function(tag, params, settings)
	if tag == "action" then
		local action_type, action_option = params:match("(.-):(.*)")
		settings.action = {
			type = action_type,
			option = action_option
		}
	end
end


.... in parse.lua below other tag definitions

	elseif M.callback then
		M.callback(tag, params, settings)
	end

[subsoap]

More complicated non-data tags would require multiple callbacks too though, how to handle them in all of the situations of RichText, but just the data part would be useful to people no doubt.

And in these cases users may want to just edit RichText itself or suggest / contribute new core feature tags.

[subsoap]

Another possible case would be a macro tag, which could be used by people to apply multiple common effects to text based on the macro name

Input
<macro=lotus>undying lotus</macro>

Ouput
<action=particles:text_emitter_red><color=lotus_red>undying lotus</color></action>