Add support for EBS volume discovery

This PR adds support for EBS volume discovery:

* Adds `--filesystem`/`--no-freeze-filesystem` option for specifying filesystems that should be snapshotted without freezing (e.g. / or non-ext4/xfs filesystems)
* If no volume ids are specified on the command line, they will be discovered using `ec2:DescribeVolumes`, `mountpoint(1)`, and sysfs based on the filesystems specified by either `--filesystem` or `--freeze-filesystem`.

Fixes #43