Add conditional attribute, fixes #65

[a-h]

This PR adds the ability to add attributes to elements using if/else statements.

package testelementattributes

templ render(p person) {
	<div style="width: 100;"
		if p.important {
			class="important"
		}
	>Important</div>
	<div style="width: 100;"
		if !p.important {
			class="unimportant"
		}
	>Unimportant</div>
	<div style="width: 100;"
		if p.important {
			class="important"
		} else {
			class="unimportant"
		}
	>Else</div>
}