Improve performance of parsing media playlists

[eric]

I tried benchmarking an m3u8 with lots of attributes and found that there were some simple places to reduce memory usage and speed up parsing.

Here's a benchmark of the difference:

before

BenchmarkDecodeMediaPlaylistWithAttributes-16    	      50	  23400226 ns/op

after

BenchmarkDecodeMediaPlaylistWithAttributes-16    	     142	   8324689 ns/op