[webauthn] new commits pushed by vijaybh

The following commits were just pushed by vijaybh to 
https://github.com/w3c/webauthn:

* Address feedback from WG review on 6/15 call

- Rename "signature extension" to "authentication extension" (will be
added to glossary in #131)
- Pre-defined extensions recommended (but not required) for
implementation by clients
- Require extensions to update either ClientData or authenticatorData,
update authenticator selection extension to obey this rule
- Clarify that authenticator arguments are sent to the authenticator 
as
a map, not as bare values
  by Vijay Bharadwaj
https://github.com/w3c/webauthn/commit/04ccbfbdf63342c974618cc2534cea5e2080e627