Support adding a Header to Ajax requests for Authentication through options

[jchomicki]

I'm trying to use rasterizeHTML to rasterize html that includes an image with src that points to a server which requires Authorization through a Bearer Token.

It doesn't seem there is currently any way to attach a Header to the requests made from rasterizeHTML through inlineresources. It would be very helpful to enable this functionality, then expose it through rasterizeHTML.

[cburgmer]

A workaround for now could be to replace the global instance of XMLHttpRequest with an implementation that provides this header. An example implementation of a proxy instance can be found in src/proxies.js of rasterizeHTML.js.

I still feel this is a bit of a stretch for the library, but understand that users might have different needs when fetching resources. A solution could be to provide a way to inject an alternative dependency for making calls. Ideas welcome.