Cloudinary widget support for JWT token

[IskIvanov]

Adding support for JWT token as custom headers parameter.

[bubbaspaarx]

I'd be interested in adding this in, however as there are no default headers set with your changes, it would ultimately break for anyone has conflicting installations of Axios, hence the use of 'instance' to set headers only for this package. If someone has a different installation of axios with global customizations, then this package would break. Hope that makes sense