Removed unnecessary liftIO.
Removed automatic error printing in System.REPL.Command.
Added error message to ParamFailure.

System.REPL.Command.paramErr no longer prints the error message, but
returns it. That message is, in turn, put into a ParamFailure by the
calling commands and thrown. This avoids double-printing of errors.
Renamed :tree to :crawler in the CLI and fixed its argument behaviour.

:crawler is now a CommandN, with the crawler name being necessary and
the URL being optional. This results in correct behaviour when the user
enters :list as the command name (i,e. the CLI doesn't ask for the URL).

It is still possible to enter ':list <URL>' as arguments, but this only
a very minor anomaly.
Added helper functions for BinaryData-type FetchResults.
Closes #26.