Updates to support SOLVE-IT-X

[chrishargreaves]

This merges the SOLVE-IT-X extensions capability into the library. This achieves a few things

• the code is much cleaner for handling extension data
• the extensions are loaded when the library loads the knowledge base
• and default data (in extension_data.json in each technique, weakness and mitigation) is added automatically to the technique, weakness and mitigation objects via the knowledge base, allowing programatic access to data within extensions.