Ability to merge databases and tables

Hi! Let me firstly say that I am a big fan of your work -- I follow your tweets and blog posts with great interest 😄.

Now onto the matter at hand: I think it would be great if sqlite-utils included a merge or combine command, with the purpose of combining different SQLite databases into a single SQLite database. This way, the newly "merged" database would contain all differently named tables contained in the databases to be merged as-is, as well a concatenation of all tables of the same name.

This could look something like this:

sqlite-utils merge cats.db dogs.db > animals.db

I imagine this is rather straightforward if all databases involved in the merge contain differently named tables (i.e. no chance of conflicts), but things get slightly more complicated if two or more of the databases to be merged contain tables with the same name. Not only do you have to "do something" with the primary key(s), but these tables could also simply have different schemas (and therefore be incompatible for concatenation to begin with).

Anyhow, I would love your thoughts on this, and, if you are open to it, work together on the design and implementation!

Start by reviewing the existing sqlite-utils CLI commands and the proposed sqlite-utils merge cats.db dogs.db interface. Define how same-named tables, primary keys, and incompatible schemas should be handled before implementing it. Done means the merge behavior and conflict rules are agreed and covered by tests for distinct and overlapping tables.