shebang support

[tobert]

Investigate whether adding shebang support is worthwhile. Implement said feature.

e.g.

yaml works out nicely since # is a comment.

#!/bin/sprok-yaml
yaml: true

Ditto toml.

#!/bin/sprok-toml
toml: true

json would be hacky, maybe skip it for that reason? Maybe follow Node's lead.

#!/bin/sprok-json
{ "json": true }