Re-add the Plugin-Evaluation

[lngr]

We need to re-add the plugin evaluation that loads an evaluation from a dynamic library. Due to problems with dlopen(), this may only be enabled on non-static builds. Most of the necessary code is already in src/plugin_evaluation.h but needs to be converted to a SequoiaInternalEvaluation that converts the assignments to a set of ints, as required by Evaluation.

Should be called on the command-line as
-e Plugin -E "-p <plugin.so> <other args>"
(see also #5 for the -E)