Add --convert option from Hoogle4

[michaelbjames]

Adds feature not ported from Hoogle4, as noted in https://github.com/ndmitchell/hoogle/issues/373.

This re-introduces the --convert flag which takes a list of text databases to index into the binary format.

This is a more strict version of --local=directory, since this will only read the specific files listed on the command line, whereas the --local option will read all .txt files, even those that might not parse.

In my use case, I don't need the hoogle server as I just use the CLI lookup. So, it seems unclear what the behavior should be when serving a text database through the server interface. I suppose it is possible to point Hoogle to the right documentation directory where it could find the rest of the Haddock docs. If you have any suggestion for this murky support for the server interaction, I'm open to fixing it.