[feature] Add supplier / SPN info

[formatc1702]

This has arisen as a personal need.

The current component attributes manufacturer and mpn are great for generating useful BOMs.
However, often it is also important to specify a specific supplier, which likely uses an independent part numbering system.

Proposal: add supplier and spn fields, analogously to manufacturer and mpn.

Obviously, it may also be desired to provide more than one supplier for a given part. That feature is beyond the scope of this feature proposal; realistically, complex supplier management should be left to a dedicated ERP system, which could map company-internal part numbers (using the pn attribute) to any number of manufacturers and suppliers.

[formatc1702]

The component nodes might get rather crowded when pn, manufacturer, mpn, supplier and spn all have values assigned. Hiding these fields in the graphical diagram, and resorting to BOM cross-referencing (as will be made possible by #226) should alleviate this.