Add a merge command

[chrisdmacrae]

Problem's we're trying to solve

In some cases, you may have many fields that are very similar or are used for the same purpose, and want to consolidate these to have leaner FMTs.

This currently requires a lot of tedious, manual work. Having a utility that finds all possible values for each key, and allows you to merge them / pick a preferred value for a single (or new key) would be ideal for site migration and updating.

• Add ability to query the values of a file by key
• Allow the specification of the replacement key
• Prompt the user for the correct value
• Allow the user to optionally specify a "winner cascade", where values don't need to be confirmed, but are used in order found. (E.g, --by-fields description,summary,excerpt)