Add attribute-parsing code to widget constructor.

Handles three cases:

1. data-mkws-config -- argument is a JSON fragment containing any
	number of key=value pairs to be added to the configuration
2. Any other data-mkws-NAME -- argument is a string, and is set as the
	value of the configuration item NAME.
3. Any other attribute NAME that is not one of a hardwired set that are
	recognised and discarded -- argument is a string, and is set
	as the value of the attribute name.

Case 1 is the industrial-strength one that can be used when setting a
complex value, such as a list of facets:
	data-mkws-config='{ "facets": ["xtargets", "author", "subject"] }'
Case 2 is the general-purpose way of setting any string value simply:
	data-mkws-perpage="50"
Case 3 is a simplifying short-cut for case 2:
	perpage="20"