Mounting single files into the guest

I would like to extend `mounts` in 2 ways:

1. Allow different host and guest locations
2. Allow mounting single files instead of directories

The first seems easy enough, but it is not clear to me if this is going to work with sshfs. Also given that #118 is WIP, this may be wasted effort (unless it is rather simple to implement).

Any hints of making single file mounts work with sshfs? And if not, will those be possible with samba?

One use case (I have others) would be adding custom CA certs to the guest:

```yaml
mounts:
- host: ~/company/root.crt
guest: /usr/local/share/ca-certificates/company.crt
writable: false
provision:
- mode: system
script: |
#!/bin/bash
update-ca-certificates
```

I guess I would like to have something similar to the capabilities of the docker `-v` option to define mounts.