jpm xpi should work with --prefs

[gregglind]

I don't see anything in lib/xpi.js that does this.

[gregglind]

jpm/lib/task.js

Line 38 in a7e0642

return setupPrefs(id, options)

is where it happens in run and test.

CLAIM: it should be during INSTALL ONLY, and happen during all the other startup stuff, IFF reason = install.

I have no idea if what should happen during uninstall or reset. Maybe that needs thinking?