Add nodeValue

I use nodeValue a lot in my app.  

https://developer.mozilla.org/en-US/docs/Web/API/Node.nodeValue

I see that nodeValue and textElement are very similar.  A query on github comes up with 2.8 million+ usages for nodeValue vs 1.8 million for textContent.  Due to the widespread use of this, I wanted to add to dom-lite.  Due to dom-lite being a lightweight library, the differences between nodeValue and textElement are so minimal, that it is moot for this library.  Therefore, the best implementation is just a reference or copy of textElement.  I chose a direct copy, because I couldn't figure out how to reference a getter or setter.  (ie nodeValue: get this.textElement()

This leads to a small amount of code duplication.  However, the other way of writing this could be done by creating two functions

function setText(){
    return this.hasChildNodes() ? this.childNodes.map(function(child){
        return child[ child.nodeType == 3 ? "data" : "textContent" ]
    }).join("") : this.nodeType === 3 ? this.data : ""
}

function getText(text){
    if(this.nodeType === 3) return this.data = text
    for (var self = this; self.firstChild;) self.removeChild(self.firstChild)
    self.appendChild(self.ownerDocument.createTextNode(text))
}

then calling them like this

	get textContent() {
        return getText.call(this);
	},
	set textContent(text) {
        setText.call(this, text);
	},
    get nodeValue() {
        return getText.call(this);
    },
    set nodeValue(text) {
        setText.call(this, text);
    },



less duplication, but also more bloat and harder to understand