Dectect unsupported features in core description files

[olofk]

CAPI2 is constantly getting new features. When these are getting used in core description files, they are not compatible with older versions of FuseSoC anymore as examplified in #32

We should look into better version managment. First idea that comes to mind is adding a minor version to the CAPI. Longer term it could be a good idea to split out the CAPI definition to its own project with validators and stuff