rss-bot selects the topic of its RSS notification messages based on the
topic of the RSS feed. Monitoring a large number of RSS feeds therefore
leads to a large number of topics in the stream. Also, the user has no
option to customize the topic names.
This patch adds a new `--topic` argument that replaces the topic for
all RSS feed notifications with the provided string. If no custom topic
is provided, the bot uses the default behaviour described above.
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