Add support for third-party features

[Twijn]

Adds features field to both shop-level information and item information. This is intended to be a more elegant solution to Klog's solution for broadcasting support for a feature, adding +klog to the software version.

Shop-level support can be overwritten per item by excluding that feature type in their features field. features should always be treated as case-insensitive.

snake_case might be my recommendation for feature naming schemes, using colons (:) to differentiate features of the same service (klog:delivery vs klog_delivery), where applicable. Special characters other than : and _ should be avoided and may be stripped by readers.

I plan to PR a markdown ShopSync spec including these recommendations

[scmcgowen]

Maybe include an example feature that uses a colon to differentiate features of one service (like shipify:receive)

[scmcgowen]

LGTM, if there's not any NACKs within 24h it'll be merged.