Add REPL context

[goodki-d]

Introduces REPL context to allow users customise their REPL experience.

Please refer to #3018

API

# variable == module/class/object/callable
app.repl_ctx.add(variable, "<name>", "desc")

Renders in console as:

<name> <type> : <desc>

[ahopkins]

I'm wondering if we should get a little smart with this and allow assignment so that in the simplest use case there is just this:

def foo():
    """Some docstring"""


app.repl_ctx.foo = foo

The .add(...) method is just when you want the more fine-grained control.

[goodki-d]

That'd be somewhat simpler to implement also a simpler API. We'll need to do formatting (like truncating doctoring to fit console) on render time, that's fine.

Should we use an internal state (like currently _locals set) or just iterate the attributes?