Turn addon-dir into an absolute path (fixes #483)
Turn addon-dir into an absolute path (fixes #483)

The addonDir option seems to only work fine with absolute paths. This is
fine as long as jpm only uses those internally and for tests, but it
breaks the `jpm xpi --addon-dir ./my/addon` command. The easiest fix is
to just run a transform function on that CLI option that makes it absolute.

johannhof/jpm,johannhof/jpm,kkapsner/jpm,wagnerand/jpm,mozilla-jetpack/jpm,wagnerand/jpm,mozilla-jetpack/jpm,kkapsner/jpm,mozilla/jpm,freaktechnik/jpm,kkapsner/jpm,alexduch/jpm,wagnerand/jpm,alexduch/jpm,johannhof/jpm,mozilla/jpm,mozilla-jetpack/jpm,freaktechnik/jpm,alexduch/jpm,mozilla/jpm,freaktechnik/jpm