This commit adds support for passing options like the following:
* iMSTK_<project-name>_GIT_REPOSITORY
* iMSTK_<project-name>_GIT_TAG

This approach enables:

* Streamline external project maintenance by allowing to develop patches
  directly in the source checkout downloaded by external project.

* Configuring the version of each external projects when integrating
  iMSTK in custom applications (e.g Slicer-based app).

Note that iMSTKData and VTK external projects have not been updated.