let url_encode filter also accept array of query parameters

BC break: no
feature addition: yes
tests pass: yes
documentation: yes

This is also supported in Jinja and so it's consistent: [jinja urlencode](http://modular.math.washington.edu/home/wstein/www/home/bjarke/sage-4.4.4/local/LIB/python/site-packages/Jinja-1.2-py2.6-linux-x86_64.egg/docs/html/builtins.html)