Add shebang support to diagrams

[ssbarnea]

By adding a shebang line to the first line of the diagrams we should be able to build them by executing them.

#!/usr/bin/env mermad

The are lots of tools that allow this use case, the trick is that mermaid should be able to parse the stdout, nothing else.

Currently it seems that the PNG files are generated if you do this but they contain an image of the source code instead of the diagram? Quite a strange behaviour.

[tylerlong]

What is mermad?

The latest tool to process mermaid code is https://github.com/mermaidjs/mermaid.cli

Closing it since it's not a common requirement.