[w3c/webauthn] 04ccbf: Address feedback from WG review on 6/15 call

  Branch: refs/heads/vgb-extension-opt
  Home:   https://github.com/w3c/webauthn
  Commit: 04ccbfbdf63342c974618cc2534cea5e2080e627
      https://github.com/w3c/webauthn/commit/04ccbfbdf63342c974618cc2534cea5e2080e627
  Author: Vijay Bharadwaj <vijaybh@users.noreply.github.com>
  Date:   2016-06-15 (Wed, 15 Jun 2016)

  Changed paths:
    M index.src.html

  Log Message:
  -----------
  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