Allow to change the prompt of a runner from command line

[eldipa]

Describe the feature you'd like
We use >>> for Python, >> for Ruby, ?: for C/C++ and we are running out of prompt symbols.

Rust for example uses >> which would be in conflict with Ruby's.

In the past we used to say arbitrary Rust example will use rs> as the prompt and it is a valid default decision.

byexample must guarantee that any file can contain and mix of languages including Ruby and Rust but that's no necessary a hard requirement for all the users.

People may just want to run a subset of languages and if they want to run Python and Rust, there is no reason to force them to use rs> instead of >>.

Proposal? Allow the user to change the prompt as we allow him/her to change the command line (shebang).