Added method TestApp.set_parser_features

In case someone needs to use a specific parser before a request, should be able to set it inside the test case. Currently, BeautyfulSoup Python’s html.parser, lxml’s HTML parser, lxml’s XML parser and html5lib, but there isn't a clean way to change the default one.
The usage should be like,
def test_something(self):
    self.set_parser_features('lxml')
    response = self.app.get(some_url)