rustyline doesn't currently support changing the prompt while in the
core readline loop.  There are a number of open PRs and issues for this
functionality, but all of them appear to be stalled for more than a
year.

Looking at kkawakam#696 and 4ec26e8, the traditional appoach to this is to
provide a reference to a trait object (`Prompt` or `ToString`), but with
that appoach there's no way to cause the prompt to be redrawn for a
change without user input.  This means for these appoaches the prompt
could change without being displayed to the user.

There's an existing mechanism to allow another async task/thread to push
input into the core readline loop, the `ExternalPrinter`.

In this commit, I expand `ExternalPrinter` to add `set_prompt()`.  With
various plumbing, this function results in `wait_for_input` to return
`Cmd::SetPrompt(String)`.

One of key change here is `State.prompt` changes from `&str`
to `String`.  There is a performance hit here from the copy, but
rustyline would need to prompt and receive input hundreds of times per
second for the copy to have a noticable performance inpact.

Added examples/dynamic_prompt.rs to demonstrate the functionality.

Closes kkawakam#417

Related kkawakam#208, kkawakam#372, kkawakam#369, kkawakam#417, kkawakam#598, kkawakam#696