Add ability to refer to subfields of objects via dot notation in string formatting

[fedya-at-db]

You can now do something like this: "%(a.b)s" % {a:{b:"hello world"}} which yields "hello world"

This is similar to python3's format strings behavior, though much simpler in capability. Every subfield must be an object except the last one.

Tested: added new happy path format test, and an error test

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[fedya-at-db]

@sparkprime Hi, wanted to check what you thought of this PR.

[sparkprime]

I only just noticed this. It seems like it could be useful for a fair few cases but it's a bit too big for the release I'm just doing.

[fedya-at-db]

At this point I've used a workaround like "%(b)s" % {b: a.b} but I'm still happy to submit this if you think it's a usability improvement.