Add symlink to last snapshot on source volume

As mentioned by @Massimo-B on https://github.com/digint/btrbk/issues/283#issuecomment-509572074 I think it is a good idea if Btrbk would create a symlink to the last snapshot in the source volume. This should of course be configurable as the where the symlink is placed and what it is called.

For example if we have this setup:
```
/mnt/rootvol/volume/
root
home
var
/mnt/rootvol/snapshots/
root.20190710T0000
root.20190710T1000
root.20190710T2000
... and so on
```
A symlink could be placed in `/mnt/rootvol/snapshots/` linke this
```
/mnt/rootvol/snapshots/
root.20190710T2000
root.latest - > root.20190710T2000
```
Another option
```
/mnt/rootvol/volume/
root
root.latest -> ../snapshots/root.20190710T2000
home
var
```
Third option
```
/mnt/bootvol/boot
boot.20190710T0000
current -> boot.20190710T0000
previous -> boot.20190709T0000 (for easy change back in grub)
```

Of course, people have different needs, so this needs to be configurable.

The issue names no source files, tests, or entry points. Start by locating snapshot creation and configuration handling, then define how configurable symlink locations and names should work; done means the latest snapshot can be referenced through the requested configurable symlink arrangements.

Written by the indexing model from the issue text.