Provide extension points for custom properties and property values

[herzbube]

For custom properties that make use of the 9 SGF data types a nice idea would be to expose the internal value type descriptor system in a simplified fashion. The main problem with this approach is that libsgfc++ cannot rely on a number of preprocessing steps done by SGFC for the well-known properties that are defined in the SGF standard. At least the following preprocessing steps come to mind, but likely there are more:

• Removal of unnecessary escape characters in SimpleText and Text values
• Line break handling in SimpleText and Text values
• Handling of invalid values for the primitive data types (Number, Real, Color, Double)

For custom properties with values that have a custom format a solution is needed where the library client can inject its own types, together with its own text-to-value and value-to-text conversion logic.

[herzbube]

When this issue is implemented, look at re-introducing support for game-specific properties. Cf. issue #35.

Also check out commit d69f639 where preliminary support for many properties was removed. It might be possible to simply re-add much of the code that was removed in the commit.