Implement Modify command

It would be great to support another kind of editing command which supports quick modification of fields across multiple entries. This could be used (e.g.) to add a tag to a filtered list of entries all at once. A possible syntax could look like the following:

cobib modify --append tags:"some new tag" -- <list of labels>

This would append "some new tag" to the list of tags for all specified labels. Combined with a filtered selection like done for the export command or through the TUI this could be a powerful and useful tool for mass-modification.

• basic implementation (!24 (merged))
• append-mode (depends on #49 (closed) and #55 (closed))