services: dicod: Allow the configuration of "handlers".

* gnu/services/dict.scm (<dicod-configuration>)[handlers]: New field.
(<dicod-handler>): New record type.
(<dicod-database>): Add fields.
(dicod-configuration-file): Support convert handlers and enhanced databases.
configuration to config file.
* doc/guix.texi (Miscellaneous Services): Update accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>