Support for local files

[beardypig]

As requested in #253 and chrippa/livestreamer#1514.

Support for local files via the file:// protocol.

hls, hlsvariant and httpstream are supported. Files can be loaded from relative or absolute paths, or even read from stdin.

eg.

streamlink hlsvariant://file://./playlist.m3u8 best
cat playlist.m3u8 | streamlink hlsvariant://file://- best
# httpstream works, but is mostly pointless :)
streamlink httpstream://file://./hadoken.mp4
cat hadoken.mp4 | streamlink httpstream://file://- best

[codecov-io]

Current coverage is 43.06% (diff: 75.00%)

Merging #304 into master will decrease coverage by 0.01%

@@             master       #304   diff @@
==========================================
  Files            43         43          
  Lines          4160       4159     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits           1792       1791     -1   
  Misses         2368       2368          
  Partials          0          0          

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[gravyboat]

Good stuff, thanks @beardypig!