Add api auth

This closes #2
Both **library** and **command-line application** now accept configurations / arguments for setting the `api_key`. 
The **documentation** has also been updated with **usage examples** and the [Filepreviews.io](http://www.filepreviews.io) url to register applications for an `api_key`.

This will be released shortly as version **`v1.1.0`**
@jpadilla