[media-types] application/vnd.taskcluster.json-e.v3+json

[Dustin Mitchell <[e-mail]>]

JSON-e is a templating language that operates on data structures,
rather than text.

In https://github.com/taskcluster/json-e/issues/319, Nicholas Bollweg
has asked for a media type that would allow Jupyter to distinguish
"plain" JSON content from JSON-e templates, and suggested
`application/vnd.taskcluster.json-e.v3+json`.

This has a `vnd` prefix, so I understand that no explicit registration
is required, but would nonetheless appreciate some advice on the
matter.

The encoding of files with this type is identical to
`application/json`.  The `v3` corresponds to the `v3` release of the
library, where major versions are incremented whenever breaking
changes are made (rarely!).

The intent would be to define this media type in the README for the
project, for use by consumers of the library.

I'd appreciate any feedback on this media-type name.

Dustin