fix seeking in ffmpeg

When playing with ffmpeg plugin and user seeks to the beginning of track, cmus
wouldn't immediately start from the beginning, but would continue playing for
a while (about 3~5 seconds), and then suddenly jump to the beginning.

A segment of code is added in ffmpeg_seek() to flush the decoded output and
reset the input buffer. This should make it jump immediately, but the current
duration after jump is still not precise.
fetch implement spawn replacement that returns it\'s output
added a read_all that actually reads ALL data available
beginnings of the lyrics view
actually compile the lyrics
fetch had a minor error resulting in a warning
add a global option for the lyric fetch command

Conflict in rebase: options.c had a struct array changed in master branch to
support some new feature. Adapting 'lyrics-cmd' to the new array.
make lyrics_fetch callable
add it to the default config
fixed minor cast warning
add a lyrics context for good measure
cosmetic correction
major segfault fixed in really_read_all
add lyrics view

The author deleted the 'lyrics-cmd' option again in this commit, for some
unknown reason. Deleting it in the new struct array similarly.
fetch remove debugs and give better output
fix mem leaks
remove debugging statements
cosmetics
moved cmd_fetch_lyrics to a more appropriate place
rework fetch command