Add a get_keys() method to ParsedTemplate class

[martin-langhoff]

For some use cases, the application using templates needs to know which keys the template is using.

For example:

• where the "natural" key names are long/unwieldy, and the user can input an easier key ("name") and then provide a mapping in a complementary UI (matching 'name' to 'foo.bar.baz.TheFrobName', picked from a dropdown).
• where resolving a key is expensive, and the scope carries many keys possible keys

So we end up using a utility function like:

def pystache_template_parsekeys(template):
    # fragile, relies on pystache internals
    parsed_template = pystache.parse(template)
    keys = []
    parse_tree = parsed_template._parse_tree
    keyed_classes = ( pystache.parser._EscapeNode,
                      pystache.parser._LiteralNode,
                      pystache.parser._InvertedNode,
                      pystache.parser._SectionNode )
    for token in parse_tree:
        if isinstance(token, keyed_classes):
            keys.append(token.key)
    # return list of unique items
    # (json does not like sets)
    return list(set(keys))

Obviously this will break if/when Pystache internals move around.

It would be much saner if Pystache had this internally, then we could do

 parsed_template = pystache.parse(template)
 keys = parsed_template.keys()