Documentation of ignore/exclude/raw

Here is a start of a documentation page for sculpin_kernel.yml I've copied some from the main documentation page and added example usage of the raw/ignore/exclude options. Most of this copy is adapted from info I got from @simensen in IRC today.

This branch is not ready to be merged yet. I currently have a @todo question in the explanation of 'exclude'. Also I'm not sure if this page should document everything that could go in sculpin_kernel.yml as that is a big list I imagine.