This commit closes #10.

- Replace custom configuration parsing and format with a YAML-based
  solution
- Improve implementation of settings handling to enable features such
  as:
  - Generally only specify defaults once in the code, not scattered
    throughout with possibly different values
  - Print comprehensive report of values used for each setting at the
    end of a run
  - Warn when a setting is specified that is not used during a run (this
    is foreseen and now possible, but not implemented yet)

NOTE: This is a squashed commit to keep the commit history of master
simple. The full history of the development branch will be retained as
10-rewrite-settings-input+commit-history in case it is needed, e.g. to
find a (small) changeset that introduced a bug using git-bisect.

A good example for new syntax is:
Examples/V_plus_Jets/LHC_WJets/Sherpa.yaml

The command line can process words using the YAML syntax, but also
supports the old legacy syntax, at least for scala...