Add no-prompt command line option (resolves #730)
lib/pry.rb,lib/pry/cli.rb: fix the "no-prompt" switch

This sets two prompts for Pry. The issue is that if you have a custom
prompt (like me), then you'll face this error:

  undefined method `[]=' for #<Proc:0x00000002f329e0>

The problem is that the no-prompt switch doesn't set the second prompt.
Also, be consistent with the codebase and store the prompt in a
constant.
lib/pry/cli.rb: rename the "no-prompt" switch to "noprompt"

Since the switch resembles the "noprompt" switch from IRB, it makes more
sense to keep the name the same; thus we avoid frustration of IRB
"noprompt" users.