Add support for adding sibling elements in a way which preserves order

[jakraft]

Problem

Currently, the only way to add a sibling element is to append a child element to its parent:

currentElement.up().ele("siblingElement");

This works great if you're building a new XML document from scratch, but it makes it impossible to update an existing XML document in a manner which preserves order. Consider the following document:

<parent>
  <child1 />
  <child2 />
</parent>

If currentElement points to <child1 />, calling .up().ele("siblingElement") would result in this output:

<parent>
  <child1 />
  <child2 />
  <siblingElement />
</parent>

Proposal

Add two new methods, eleBefore and eleAfter, which would add a sibling element before/after the current element.