Multiple changes to the main file, after this unisync becomes kind of
usable.
Add subcommands : this uses the 2 previous commits to add the
subcommands to unisync it is now possible to sync, add and mount.
pathlib : move from PosixPath to Path
Remove unused imports
Rename base_namespace to cli_args
Add some comments and TODOs
this adds subcommands to the argparser using subparsers, we also set a
default value for func depending on which of the subcommands is
selected.
Also change the formatting of the epilog so it is on two lines.
This adds runners.py, it contains a set of functions that peform all the
various task that unisync can do (sync, add and mount for now).
They are simple function that put together all the rest.
