Cobalt serve does not monitor _cobalt.yml

[Geobert]

If we change values in _cobalt.yml it is not picked up by cobalt serve.

I've change default: excerpt_separator and it was not reflected.

[epage]

The nice thing is we are at least on-par with other static site generators, like jekyll though I don't know if we have this documented.

As I roll out more of the collection refactors, this should be easier to do.