Allow separate support for ie6 and/or ie7 in clearfix mixin

Right now you need to specify support for IE6 *and* IE7 if you want to output the legacy or micro clearfix.

So in a context where:
<pre>
$legacy-support-for-ie6: false;
$legacy-support-for-ie7: true;
$legacy-support-for-mozilla: false | true;
</pre>

You always get the modern one.