Support sync from and to files

[bomoko]

In #67 we introduced the ability to fake "sync to file" functionality by skipping deletion and import of the temporary transfer files.
Now we'd like to make this a full fledged part of lagoon-sync with its own command set.

To this end, we'd like to have some set of commands of the form lagoon sync to-file and lagoon sync from-file that allow us to specify target and source files to be dumped/imported from the various synchers.