Add support for other programming languages and main document formats

[nuest]

ERCs would need some kind of configuration mechanism then, e.g. in erc.yml language: R. Tooling support could probably very well detect other main document formats, such as Jupyter Notebooks.

Potential languages which are supported by R Markdown and could already work now:

• Python
• SQL
• Bash

Further languages

• Octave (seems to be supported by knitr, but not documented: https://github.com/yihui/knitr/pull/1244)
• Go
• Julia