Add Unichem support

Unichem provides access to many different identifier and is an EBI resource, so should be reliable.

https://www.ebi.ac.uk/unichem/search/connectivity?type=inchi&compound=InChI%3D1S%2FC10Cl12%2Fc11-1-2%2812%297%2817%294%2814%293%2813,5%281,15%299%287,19%2920%296%281,16%2910%2821,22%298%282,4%2918