Fix some compiler warnings.
Print error messages in colour when compiling with Clang and Ninja.
Fix #51 - Improve MIDI playback of muted notes.

They are now played as very short staccato notes, instead of just
switching over to the "Electric Guitar (Muted)" patch for that note. The
muting effect sounds a bit more noticeable than in v1.7, but it sounded
good with the MIDI soundfont I'm using on Ubuntu.