Show $SHLVL in prompt if greater than 1

[chris-marsh]

I think it would be useful to have the prompt show the shell level.
This is probably more useful when you messing about with things like PureLine dev :) I'm constantly sourcing pureline or starting new bash sessions to test stuff. Working my way back with Ctrl-D is tricky to know when your at ground zero.

Prompt segment could be

|$> bash
|2$> bash
|3$> bash
|4$> ctrl-d
|2$> ctrl-d
|$>

[hamza-arshad]

@chris-marsh can I pick this?

[CyrilOtheninGirard]

See pull request #80 for a possible implementation