Allow to control vimb by fifo file (#144).
Fix segfault on quit by control fifo (#144).

If vimb is closed by the fifo with `echo ":q\r" >> {fifo}` or `echo '<C-Q>' >>
{fifo}`, vimb crashed because of a too early cleanup during allready active
main loop.
The cleanup code is now called right after the main loop is killed.
Merge branch 'feature/fifo'.