added support to apply filters from the URL

To set the input from the URL, set the variable "input" to your input.
To set any of the filters to true, set the variable of their name (i.e.
"windows", "steam", etc) to any value.

For example, if you wanted to only search for DRM-Free Windows games
containing the phrase "blendo", your URL would be
"index.html?input=blendo&windows=1&drm-free=1".