Extend Kernel#p with power_assert.

Note that power_p works only if p is written in the file.

$ cat test.rb
p { 3.times.to_a }

$ ruby -rpower_p test.rb
p { 3.times.to_a }
      |     |
      |     [0, 1, 2]
      #<Enumerator: 3:times>