currently upon calling Pry::Prompt[:foo] a new Hash is created, and on all
subsequent calls a new Hash is also created. it's wasteful and careless,
so let's fix it.
Fixes#1738 (Possible to make prompt_name dynamic?)
The user-facing API is the following:
```rb
Pry.config.prompt_name = Pry.lazy { rand(100) }
[1] 80(main)>
[2] 87(main)>
[3] 30(main)>
```
Since we always need to define two procs that look almost the same, duplication
was unavoidable. With help of method wrappers around procs we can reduce it.
As a bonus, the class has some YARD annotations now.
It makes a lot more sense to keep these procs under the `Pry::Prompt` namespace
than `Pry`, which is already heavily populated by other various things.
the list-prompts command shows a list of available prompts as well
as the active prompt in the list. prompts are drawn with a name as
well as a short description.
change-prompt can be used to change the prompt by the name found in
the list-prompts command.
