configuration from the filesystem

In the documentation, Confit mentions taking configurations from directories. I'm guessing that means it expects a file of a prescribed name in those directories. I would rather just have a function that takes a path and returns a configuration that I can pass to `override` (#9). Then, composing a colon-separated path of configuration files in precedence order turns into this:

``` python
reduce(confit.override, map(confit.from_file, PATH.split(':')))
```

I think there is a good opportunity for directory based configuration, though. Many projects (like the users of [this other confit](https://github.com/krakenjs/confit)) want to use different sets of defaults based on different **environments**. Common environments include "development", "qa" or "staging", and "production". It would be convenient if they could have a directory of multiple configuration files named after the environments.

``` python
def from_file(env, path): # env comes first for partial application
file = path
if not file.is_file():
file = path / env
if not file.is_file():
file = path / (env + '.yml')
... # more guesses
with open(file, 'r') as stream:
return yaml.load(stream.read())
```

Start with the documentation's discussion of directory-based configuration and inspect the existing override API referenced by issue #9. Evaluate the proposed from_file(env, path) entry point, including direct paths and environment-named files, and define completion around loading the selected configuration for composition with override.

Written by the indexing model from the issue text.