Tabs and double quotes not parsed correctly in authorized_keys file

[steepnought]

authorized_keys files allow tabs as separators, backslash-escaped double quotes in option values and require double quotes around option values.

These cases are not treated correctly by sshpubkeys.AuthorizedKeysFile. See attached example file for test cases.
ak.txt

[ojarva]

Feel free to create a PR to fix that. I'm happy to review and merge if test cases are included in the PR.