Users can run tests on different filesystems/mount points
Users can run tests on different filesystems/mount points

Add the `-dir` flag which enables the placement of bonnie's files; this
allows the user to specify which filesystem is placed under test, e.g.
`/tmp`, `/Volumes/USB`, `/var/vcap/store`, `D:\`.

There is a small bug where it may leave a dangling tempdir. The tempdir
is create before `flag.Parse()` is called, and if parsing fails then
`bonniego` will exit without running the `defer` blocks, which removes
the temporary directory.

cunnie/bonniego,cunnie/bonniego