Add disabled prop

[thedeany]

It would be nice to be able to disable the component if needed. I'm currently attempting to implement it but have it disabled until another form element is completed. I can do this directly to the child <button> element which effectively makes the button look disabled, but you can still click it to invoke the file uploader.

[travisdmathis]

@thedeany thank you for the submission, I have merged, and deployed a release to NPM.