Configure Flask app using tarbell_config

[eyeseast]

Building on the discussion from #287, Flask extensions (including Frozen Flask, which Tarbell uses) often rely on settings in a config file. For example, setting FREEZER_IGNORE_404_NOT_FOUND = True is often useful in development. Or as in #287, setting thumbnail source and destination directories.

We can tell Tarbell's underlying Flask app to configure itself based on tarbell_config like so:

site = TarbellSite(path)
site.app.config.from_object(site.project)

Or, really, do that in TarbellSite.__init__.

This doesn't deal with registering Flask extensions themselves, but it makes the process less repetitive.