Allow passing in a custom path to the tarsnapper binary

[jeffwidman]

tarsnapper expects tarsnap to be in the PATH, but that's not always true, for example under cron (#18).

So it'd be nice to be able to specify a custom path to the tarsnap binary, rather than having to always remember to modify PATH

[miracle2k]

We already have -o for tarsnap options, I could see a --tarsnap-bin or --exec argument or something similar.