Implement Support For Site-Specific "Helpers"

Hi Bastian,

I'm finding the use of site-specific helpers to be very handy, both in terms of encapsulating frequently-used functionality that doesn't belong in a plugin, and just tidying up my templates.

The biggest sticking point is that--at present--the helpers need to be manually required in each template.

The modifications in this pull request allow Kirby to automatically load any site-specific helpers from a new `/site/helpers/` directory. If no such files exist, Kirby just continues as normal.

It's worked very well for me thus far.

Cheers,
Stephen