Remove direct reference to ns.adobe in code

https://github.com/adobe/jsonschema2md/blob/60d9211f034e44e9727e8f8be2ce547ca191d4e1/lib/header.js#L14

`lib/header.js` includes a direct reference to an adobe URL.
If this tool is to be used by the community (which I understand is the intent moving forward), this is probaby not something that should remain, and should in stead be relocated to a configuration file...

Related: https://github.com/adobe/jsonschema2md/issues/112

Start with lib/header.js at line 14 and inspect how the Adobe URL is used when headers are generated. The work is done when that direct URL is no longer hard-coded and the replacement can be supplied through configuration, while the existing behavior remains covered by the relevant tests.

Written by the indexing model from the issue text.