Add support for multiple codedb-directories to serve.

[noj]

One instance of cdb serve should be able to serve multiple cdb-paths.

For instance:

cdb serve ~/src/dir1 ~/src/dir2

This would expose the following URLs:

http://localhost:port/       # List of available cdb db (possibly search all, dropdown to choose specific db)
http://localhost:port/dir1   # Search using the db in ~/src/dir1
http://localhost:port/dir2   # Search using the db in ~/src/dir2

[kalven]

I'll put this on the todo list.