default system message mentioning R

[moodymudskipper]

By default ask() is completely vanilla, I thought it was a good idea but we come to expect that ask() understands that we're doing R (it often understands through context but not always). I also want to make sure it doesn't suggest python code when it could be done in R.

So we need to tall ask :

• That we are R users
• That we prefer R chunks if code is relevant in the answer
• That it's also perfectly fine to answer non R related questions

And a documented way to make it perfectly Vanilla, maybe with context = NA

[moodymudskipper]

maybe we need a default_context arg, linked to an option so users can adapt to their taste