Add new bindings to compete with plaster_pastedeploy.

[mmerickel]

We should talk about creating a new file format around yaml and / or toml, possibly with some jinja2 templating to support macros and environment variables. The most useful thing to get this started on the right foot would be to throw out some example file formats that we can iterate on.

[sheilatron]

In addition to YAML, it might be interesting to see support for HJSON or HOCON. Here's an interesting commentary comparing these formats.

http://blog.ometer.com/2015/09/07/json-like-config-a-spectrum-of-underoverengineering/

[kpinc]

See also a pyramid-cookiecutter-starter branch: https://github.com/kpinc/pyramid-cookiecutter-starter/tree/yaml_config

(This branch should work. But I've not submitted a PR because the patch is dependent upon PRs to plaster, plaster_pastedeploy, and plaster-yaml which add a "raw" parameter to Loader.get_settings().)

Regarding templating a yaml config. I say "yes". But, I'm unclear on how to tell the loader what templating system to use. Because I'm unclear on URI syntax. It seems like the scheme is "file+extension". Would there have to be a different extension for yaml files templated with jinja2 and mako, and not templated? This kind of makes sense, but is that what the plaster URI design has in mind? Then the loader would have to look at the "extension" part of the fragment to decide how to resolve templating, or not.