add text-decoration to allowed css style attributes @@filter-controlpanel

By default, TinyMCE allows strike-through editing (next to bold and italic). This done with "text-decoration: line-through;". However, as soon as you save the edit, this is lost due to the very restricted html filters of plone. filters should be in sync with TinyMCE
Merge pull request #18 from jnachtigall/master

add text-decoration to allowed css style attributes @@filter-controlpanel