Added 'scaffold' as possible file extension

Changed the regular expression to support template files with a
.scaffold extension, this ought to simplify the use of Scaffold as view
engine in an express project.
Fixed too greedy cleanup and rendering placeholder template

Template
* The XPath queries used to find text nodes from which to remove excess
whitespace now incorporate html and body offsets which prevent sub
templates to be seen as ‘not in body’ (where all text nodes which are
left empty after a trim are removed as these are not significant)
* Placeholder values are no longer treated as pure string replacements,
if a DOM* element is provided, all of its childNodes will be inserted
at the placeholders position