[context] allow positional context passing

[signebedi]

Allow user to pass positional information as a tag to :context and to questions, see:

...
:context[a:b] - show context history with optional index ranges a and b *under development*
...
To pass context positionally, prepend questions with tags like `[a:b] what is the meaning of life`.

We will need to modify the context.py logic to stringify the index range.