Add option to discover activities from Redmine installations for configuration

[bjoernmartin]

In order to configure mappings for activities, the configuration file requires knowledge of the internal IDs of activities to map them to labels, see for example

buchungsstreber/example.config.yml

Line 11 in 98f7925

8: [DesignView, Design, View, Dsgn]

Since "normal" users don't have access to this kind of information and may not know how to scrape that out of Redmine's web interface, it would be nice if buchungsstreber were able to extract those from a configured Redmine installation (e.g. via command line argument dump-activities or similar) so the user can use them for their own configuration.

An alternative approach could be to allow the use of activity names instead of IDs. In this scenario my config might change like this:

# from
    activities:
      33: [Marketing, mkt]
      46: [Sales]
      47: [Consulting, con]
# to
    activities:
      Marketing: [mkt]
      Consulting: [con]

Marketing, Sales and Consulting are the "original" labels within the Redmine installation and will always work, hence no configuration for Sales, as no alias exists.