Fix headline justification in the RTE

Documented here: http://issues.umbraco.org/issue/U4-5409

It seems at some point when going from the old TinyMCE to the new TinyMCE the justification buttons evolved from using the `align` attribute to the `style` attribute.

To fix alter the `tinyMceConfig.config` to allow for `style` attribute on all headline tags.