allow non-singleton usage

the `register()` method keeps whatever was registered in memory, but if two libs were using `filing-cabinet` at the same time, they may have differing requirements for custom resolvers. maybe a resolver for `.foo` is different from one to the other, or perhaps one of them should not, under any circumstances, resolve a `.foo` file.

a way to address this would be to provide a factory function, e.g.:

```js
const filingCabinet = require('filing-cabinet');
const cabinet = filingCabinet.create();
cabinet.register('.foo', myFooResolver);
cabinet({partial: 'bar', filename: 'quux.js', directory: '/some/path'}) // works with .foo files
filingCabinet({partial: 'bar', filename: 'quux.js', directory: '/some/path'}) // does not work with .foo files
```

I can send a PR to that effect, or if you have a different idea...