Feature request: drop the JSON encoding requirement

[dolmen]

The HTTP service should be able to handle a raw Mermaid file POSTed with the proper Content-Type header without requiring to wrap it in a JSON document first.

[Schachte]

I'll add this. Adding a simple UI right now, so ideally the user can just drag and drop a file and get an embed link back from azure/s3. I'll add this support.