Add parameters support.

* src/cuirass/parameters.scm: New file.
* Makefile.am (dist_pkgmodule_DATA): Add it.
* bin/cuirass.in: Add "parameters" argument.
* src/cuirass/base.scm (read-parameters): New procedure.
* src/cuirass/notification.scm (build-details-url): New procedure.
(notification-text): Use it.
* src/cuirass/rss.scm (build-details-url): New procedure.
(build->rss-item): Use it.
(rss-feed): Remove "base-url" argument.
* src/cuirass/remote-server.scm: Add "parameters" argument.
* src/cuirass/zabbix.scm (%zabbix-uri, %zabbix-uri, %zabbix-password): Remove
them.
(zabbix-request, zabbix-login): Adapt accordingly.