Add a shorthand for pattern/URL settings to ConfigContainer

[The-Compiler]

Currently, there are two ways to set settings for an URL:

config.set('content.images', False, '*://example.com/')

with config.pattern('*://example.com/') as p:
    p.content.images = False

And in qutebrowser code, there's config.instance.get_obj(..., url=...) and config.instance.get_obj_for_pattern(..., url=...).

I've been discussing with @toofar about possible alternatives which would make both easier (and more consistent with the shorthand for global settings). So far, two things came to mind:

• Having subscripts on ConfigContainer - in config.py: c[pattern].content.images = False; config.val[url].content.images`. However, that can't tell URL/pattern apart and might be a bit much magic.
• Having url(...) and pattern(...) methods on ConfigContainer: c.pattern(...).content.images = False and config.val.url(...).content.images (or maybe for_url and for_pattern).

Not sure yet about what to do (if anything at all) and what'd happen with the old syntax - just leaving this here as some food for thought.