* nodes: add a `default` param to Template.get

Similar to dict.get, Template.get with a default param supplied will
return that value instead of raising an exception. If default is unset,
Template.get will keep its previous behavior and raise an exception.

* nodes: Add __getitem__, __setitem__, and __delitem__ to Template

These are just aliases for existing methods, without the ability to
specifiy additional parameters. However, including them makes Template
more dict-like, so it's a good idea to have them.

* nodes: Use def instead of assignment of a lambda in Template

Per PEP8, there is no benefit to using a lambda here, and some
downsides. It's the same number of SLOC either way, so might as well
change it.