Ignore blank lines in playlist -- fixes panic w/EXT-X-MAP.

[ejohnst]

reader.go panics if there's a blank line between the header containing EXT-X-MAP and the first segment for FMP4 media playlists. (It tries to add MAP to the first segment, which it hasn't created yet.)

Patch fixes this by ignoring blank lines; reader_test.go updated to work with change (one of the tests assumed prior behavior).

@eric confirms this is consistent with the standard:

Lines in a Playlist file are terminated by either a single line feed
character or a carriage return character followed by a line feed
character. Each line is a URI, is blank, or starts with the
character '#'. Blank lines are ignored. Whitespace MUST NOT be
present, except for elements in which it is explicitly specified.

Issue seems to have been previously reported as #166.