Support custom import/export paths

[schmunk42]

Coming from docker/machine#23 (comment) I am thinking about including machine-share in roj, with a having a generic method to export import machines, I (we) could solve the problem of making machines, swarm and stacks really portable.

But I'd need a to specify custom import/export paths around here:
https://github.com/bhurlow/machine-share/blob/master/import.js#L11-L12

[bhurlow]

@schmunk42 OK! agreed those should be configurable paths and not hard coded