A Url pattern to preselect a category

[GoogleCodeExporter]

In as much detail as possible, please describe what you would like to see.

1. A Url pattern to preselect the category
2. Using the category url should have the same view results as selecting the 
category from the dropdown list
3.

What operating system are you using?

Please provide any additional information below.

It's possible to view a specific exploration with a URL

 eg Three Balls is
https://www.oppia.org/explore/4

I would like to be able to pre-select a category with a URL

 eg something like this to show all Architecture explorations
https://www.oppia.org/category/Architecture

Original issue reported on code.google.com by [email protected] on 20 Jul 2015 at 8:12