Support stackErrors option per element / per guard

[betesh]

For example:

<input ... data-guard="required password long-password has-at-least-one-number"
    data-guard-stack-errors="password long-password" >

If required fails, it shouldn't display the other error messages, but if required passes and password, long-password and has-at-least-one-number all fail, it should display error messages for password and long-password but not for has-at-least-one-number.