Add utility functions to retrieve specs to the package

[tidoust]

The index.js is not included in the published package. On top of potentially being useful for #246, it could also be useful to export utility functions to retrieve a spec out of a shortname or a URL as discussed in: w3c/webref#115 (comment)

[tidoust]

Also see the Spec identifiers section, just added to the README, that describes an algorithm to retrieve a spec entry from browser-specs from its current shortname or from a past shortname. It would be useful to expose that in index.js as well.

Also, is seems worth checking how web-features and BCD retrieve specs they are interested in from browser-specs, and align the functionality in index.js as needed.