Add FileInput Component

This release adds the FileInput component that makes dealing with file inputs simpler. This component is the default "file" input when using FormRow. Also, due to the nature of file inputs, this is an uncontrolled component.

Example usage:

// as a plain input
<FileInput onChange={files => console.log(files)} /> // files is a FileList object

// or, in a FormRow
<FormRow type="file" />

Learn more about files and the Files API here.