Add disabled option to repositories list

With the US government shutdown I have to disable catalog.data.gov
again. To avoid the annoying commenting in and out pattern I have
added a disabled option.

With this commit I have also overhauled the main script. Now a list of
repositories can be passed to the repositories option.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix faulty pagination arguments

The arguments that were passed to perform the CKAN pagination were just
plain wrong. The starts parameter does not take a "page" argument, but
the actual offset in number of datasets.

Also the last query's rows argument is truncated to the remaining number
of datasets. This fixes the problem with the Swiss data portal. Instead
of 1630 datasets the harvester retrieved 2000 datasets including 370
duplicates.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add the Canadian open government portal

Apparently, the Canadian open government data portal uses CKAN, too.
This is a great replacement for the US data portal. Also I have added
the follow location to the dump retrieval.

While data.gc.ca offers a dump download, too, it seems too cumbersome
to deal with their JSON line format now.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>