Add random command

This command simply prints a random URL. A tag may be specified to limit
the search to that tag. In the Conkeror module this command immediately
loads the random bookmark in the current buffer.

- js/clark.js: If `error' is empty, return the value of `data'. This
  makes it easier for some commands to use the result of the command.
  Add `clark-random' to `clark_keymap'.
Move most queries to queries.lisp
Use parameter instead of constant for version

The `defconstant' form generates an error when compiling, that the value
is not `eq' to the previous value.