Rename decode → tttool

This closes: #45
Do not export invalid play commands

instead, simply skip them, but still print them in `tttool scripts` and
other commands. This fixes #46.
Merge remote-tracking branch 'upstream/master'
tttool assemble: Also look for mp3 files

This implements part 1 of #47.
GME-Format: fix internal Link
Merge pull request #50 from kne1p/patch-1

GME-Format: fix internal Link
Merge remote-tracking branch 'upstream/master'
Corrected media offset and length type