Add VSS support to Windows version

**For now this issue should be considered 'open to discussion'!**

VSS Support should be added to support 'advanced' backup features on Windows (SQL Server, Exchange, ...).

**This should**
- be rather transparent to the user (--use-vss command line option or similar, see #983)
- have the ability to report VSS error messages
- have the option to continue even if VSS creating fails

**Things to consider**
- Should this be implemented by wrapping borg (scripting VSS is quite cumbersome)?
- Should some cross-platform support be implemented (VSS, LVM, QEMU Snapshots, others)?

Edit: I'm planning on opening a bounty for this once we agree on how to implement it.