Do not crash when silk app is not included in urls

[asalahli]

Hi,

Right now SilkyMiddleware raises NoReverseMatch when silk.urls are not included in the app. There are various reasons when one might not want to enable the Silk UI, but still keep the middleware around. For example, to disable Silk in production, the easiest way would be to set SILKY_INTERCEPT_PERCENT to 0, and not include the URLs. Unless there's a reason to keep the exception in place, I'd like to have Silk handle this gracefully.

This is my first time posting here, so I'm not too familiar with the development process. Happy to address any feedback.

Thanks