Add possibility to define custom handler after successful upload

[donarus]

It would be great to add custom handler for 2xx response, when upload was successfull

usage:

<XHRUploader ... onSuccess={...} ... />

[harunhasdal]

@donarus that's a good idea. Would you like to submit a PR? Otherwise it'll need to wait a bit.

[donarus]

Hi, I think my current knowledge of React and JavaScripts is too low (haven't seen JS for > 5 years, I'm not the "frontend GUI" :-) ), but I'll give it a try this week if I'll find some time.