[webauthn] new commits pushed by vijaybh

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

* Adding fido appid extension (#229)

* Adding u2f appid extension

Closes Issue #216

* Adding updated references

* addressing comments from issue peanut gallery :)

* typo fix

* address vijay's comments
  by Alexei Czeskis
https://github.com/w3c/webauthn/commit/2d6a7a52f87fcf36bdb7df3d6e0180adae1c97d6