Implement pry-like ls command

[k0kubun]

I implemented pry's ls command for irb. This is useful when you want to inspect constants, methods, and variables of self or an object without typing a lot.

$ irb
irb(main)[01:0]> ls ERB.new('test')
ERB#methods:
  def_class         def_method        def_module        encoding          filename          filename=         lineno
  lineno=           location=         make_compiler     result            result_with_hash  run               set_eoutvar
  src
instance variables: @_init  @encoding  @filename  @frozen_string  @lineno  @src

[aycabta]

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

Rebased