Make some tweaks

[yumminhuang]

Thanks for providing such useful tool.

When I use your tool to backup EBS volumes, I found two problems:

1. It's inconvenient to add a large amount of volumes by running command one by one;
2. It's difficult to remember volume IDs. It would be better to use name tags.

To solve these problems, I modified your code:

1. Support using tag:Name to filter volumes;
2. Add new options --watch-file and --unwatch-file to read volumes from a configuration file.

I also add a new option --snapshots to list all snapshots for a given volume id or volume name.

I also updated README.rst and it shows how new options work.

[sebdah]

Thanks for this PR, it looks really good. I'm gonna read it through and test it before merging it. Will post back here as soon as possible.

[sebdah]

This has now been merged and released in version 0.4.0. Thanks again for this pull request @yumminhuang