Warn when DASH offers higher quality, misc other changes

Most notably, warn the user that youtube-dl isn't getting them the highest quality when DASH offers higher video qualities than the currently selected 'best', as happens normally when the user doesn't specify a format.

Aside from the DASH warning, no really big changes here, just some general cleanup of the messy format listing output and the code that generates it, grouping the YouTube IE's `_formats` entries by type, and correcting the listed bitrate for format 171.

As with my last pull request, I'm still not really sure how best to use git, so if I should do something differently, let me know.