Would be helpful to have support for unrecognized extensions

[dreemkiller]

As currently written, the remember_extension function only keeps track of extensions that are explicitly recognized and deemed useful.
This leaves a user of the webpki APIs unable to access any of the other extensions in a certificate, even if they just want the raw data from the extension.

It would be useful to store the 'unrecognized' extensions in case the API user needed access to them