Feature for consideration: optional prompt character(s). #16

Would it be reasonable to add the ability to define a prompt string that would be displayed whenever the shell was waiting for user input? I would typically install something like "> " or "$ ".

Why? Two reasons:

1. It makes the shell a bit more user-friendly, since it lets the user know when the system is ready to accept input (as opposed to still crunching on the previous command). This pattern is used by the standard UNIX shells.
2. Makes it easier to write a parser for serial automation of a device that uses the shell. The prompt would serve as a delimiter that indicates that the output of the previous command is finished and that a new command can be sent. This would be helpful for high-level system tests where a device is being controlled via its serial port.

I'll submit a PR for this if the idea is acceptable, but I want to check this first.