Refs #31 - Adds render() for rendering template strings in set_trace

As demonstrated in the docs changes, this function is used inside of a
set-trace for the purpose of testing out template rendering of a given
string. For instance, its user might type the following

ipdb> render("""
{% if something %}
    {{ varname.attr|filtername:"arg" }}
{% endif %}
""")

This passes the string through a template.Template and renders it with the
current context.