Add -i option to pry-rescue

Addresses https://github.com/ConradIrwin/pry-rescue/issues/56 and https://github.com/ConradIrwin/pry-rescue/issues/57.

@robgleeson @ConradIrwin I'd love for you guys to take a look at this.

What I've done:

  * Move from Kernel.load to Kernel.eval
  * Compensate for losing script-level locals by evalling everything in TOPLEVEL_BINDING (perhaps dangerous? Can we make a sandbox binding?)
  * Address syntax errors by just printing them when we see them. As we can't give a meaningful stack trace, don't trying to open a binding at the call site (there really isn't one).
  * Hook `Kernel#at_exit` et al when the `-i` flag is specified. Enables some userful behavior:

`rescue -i test.rb` yields:

    Run options: --seed 42032 
     
    # Running: 
     
    . 
     
    Finished in 0.001498s, 667.4204 runs/s, 667.4204 assertions/s. 
     
    1 runs, 1 assertions, 0 failures, 0 errors, 0 skips 
    [1] 2.0.0 > Foo.new.bar 
    "whee" 

`test.rb`:

    require 'minitest/autorun'

    class Foo
      def bar
        "whee"
      end
    end

    class TestFoo < MiniTest::Test
      def setup
        @foo = Foo.new
      end

      def test_that_foo_can_bar
        assert_equal "whee", @foo.bar
      end
    end