Add whereami command

[k0kubun]

This is another command in pry. When you execute a lot of expressions after your binding.irb invocation, sometimes the source code of your context may go out of your screen. If you use this whereami like gdb's list, you can show the code again.

$ ruby /tmp/a.rb

From: /tmp/a.rb @ line 3 :

    1: a = 1
    2: @b = 2
 => 3: binding.irb

irb(main)[01:0]> a
=> 1
irb(main)[02:0]> @b
=> 2
irb(main)[03:0]> whereami

From: /tmp/a.rb @ line 3 :

    1: a = 1
    2: @b = 2
 => 3: binding.irb

=> nil

[aycabta]

Please don't contain merge commit and please use git rebase origin/master.

Rebased