Use `<ko ...>` for virtual elements

[brianmhunt]

Since JSX doesn't allow comments, I'm adding support for a new element-type:

<ko binding='value'></ko>

becomes, in preprocessing

<!-- ko binding: value --><!-- /ko -->

[mbest]

For completeness:

<ko binding1='value1' binding2="value2"></ko>

becomes, in preprocessing

<!-- ko binding1: value1, binding2: value2 --><!-- /ko -->