Add discover.py to discover users and videos

This creates a new item type, "user", which will fetch a list of discovered users and videos (both highlights and past broadcasts with more than 100 views).

We could import this list of users to start us off: https://github.com/ArchiveTeam/twitchtv-items/blob/master/seed/channel.txt

Tested with `python discover.py puncayshun test.txt.gz`:

    $ gunzip -c test.txt.gz | grep ^user: | wc -l
    27883
    $ gunzip -c test.txt.gz | grep ^flv: | wc -l
    107
    # a random sample...
    $ gunzip -c test.txt.gz | shuf | head
    user:sleepyseal
    user:coilerr
    user:touchtheclouds
    user:ironcross26
    user:cutmyteeth
    user:soapgoat
    flv:c4840664
    user:swaggdocta
    user:jackthestripperdd7
    user:godjesusitshim
    $ gunzip -c test.txt.gz | tail
    flv:c4173427
    flv:a550640830
    flv:c2422934
    flv:c3590620
    flv:c3966433
    flv:c4802722
    flv:c4212438
    flv:a523859418
    flv:c2683349
    flv:c3431580

(Administrative note: I'll be at work for the bulk of tomorrow, so don't require my permission to make changes.)