New command merge

[aawsome]

This PR adds the merge command which allows to merge multiple snapshots.
It also fixes a minor bug how multiple paths were saved using #458.

The merging strategy is as follows:

• If files only exist in one of the snapshots, add this file to the newly created one
• If files with identical names but different content exist, take the file with newest mtime.

closes #43

TODO:

• compute correct "paths" of merged snapshots
• compute correct summary of merged snapshot, e.g. #files #dirs, total size, etc..
• enable setting other snapshot info, like tags, label, ... (use default if all are identical?)
• add --json flag
• add --delete flag to remove original snapshots