Add option for ACL

[njj]

This gives the ability to pass other ACL parameters i.e. public-read, while still being backwards compatible and falling back to private if no option is present.

[seanadkinson]

options.ACL || 'private' would be good.

[seanadkinson]

Thanks for the PR. Mind adding this option as an example to the README.md? Something like:

app.use('/s3', require('react-s3-uploader/s3router')({
    bucket: "MyS3Bucket",
    ACL: 'private'  // this is the default
}));

[seanadkinson]

Pushed version 1.1.10 with this update. Thanks!