Support pos attribute for add

[donmendelson]

XML patch standard defines attribute pos to control position of an added node. The default is append but the attribute allows prepend (becomes first child node) or before or after a specific node as specified by an XPath.

[kleihan]

This is required in order to use xml-diff-merge to build FIX Extension Packs updating the Orchestra XML file of FIX Latest. Field positions in messages, component and groups are defined as part of this process. Whilst only relevant for display purposes in the case of messages, the sequence of fields is mandatory inside components and groups for the tagvalue encoding. I support this enhancement.