Added engine for Stan
Use chunk label for variable name of modelfit object returned by eng_stan

Incidentally fixes the R CMD check error:
```
eng_stan: no visible binding for global variable ‘model_name’
```
eng_knitr: requires specification of a return variable name

- This effectively reverts commit
  d4f37499b03bbf2e1471b145ed615ef0b9401869. Chunk label should not be
  used to define variables.
- Instead of falling back to using model_name for the stanmodel object
  name, require that the name be specified and throw an error if not.
Merge pull request #903 from jrnold/stan-engine

Added engine for Stan [ci skip]