Added new constant to WintermuteVersioning.

  In order to easily get a full version string, use WINTERMUTE_VERSION.
Use WINTERMUTE_VERSION in source build.

  * With the newly defined WINTERMUTE_VERSION constant, define the version of
    the executable in CMake.
Added basis for middle-man of plug-ins.

  * Added 'pretty header' for Wintermute::PluginInterfaceable.
  * Added source and header for Wintermute::PluginInterfaceable.
  * Began writing out documentation and sources for
    Wintermute::PluginInterfaceable.
Added new files to build.

  * Added new source and header files to build.
  * Added pretty header to installation process.
A bit of formatting to the source CMakeLists.txt project.
Repaired build suite.

  - src/Wintermute/plugininterfaceable.cpp
    * Added basic logic into source.
  - src/Wintermute/plugininterfaceable.hpp
    * Defined `Wintermute::PluginInterfaceable`.
Added private header.
Added documentation to Wintermute::Plugin.