enable "webassets.cache" to override and auto-create default path to cache directory

the current "webassets.cache" configuration does not auto-create the cache directory if it overrides the default value - this patch allows it to auto-create that directory if it does not exist (just like the default directory is auto-created) if `cache` is True.

it also augments the "true/false" checking to use the full set of boolean values supported by `pyramid.settings.asbool()`.