Add setting ldap.usernameField

This determines which ldap field is used as the username on
HackMD. By default, the "id" is used as username, too. The id
is taken from the fields `uidNumber`, `uid` or
`sAMAccountName`. To give the user more flexibility, they can
now choose the field used for the username instead.

lib/config/default.js

@@ -96,6 +96,7 @@ module.exports = {
     searchBase: undefined,
     searchFilter: undefined,
     searchAttributes: undefined,
+    usernameField: undefined,
     tlsca: undefined
   },
   saml: {