text: Title: Add support for SKIPPING cells' execution
  Body: Hey,
  First, great open source project.
  
  I suggest the following feature:
  Currently to execute the entire notebook we can do:
  ~~~python
      def test_notebook(self):
          notebook_path = "fuse_examples/tutorials/hello_world/hello_world.ipynb"
          # Execute the whole notebook and save it as an object
          with testbook(notebook_path, execute=True, timeout=600) as tb:
  ~~~
  
  Now, if we want to execute only the first cell, we shall do:
  ~~~python
      def test_notebook(self):
          notebook_path = "fuse_examples/tutorials/hello_world/hello_world.ipynb"
          # Execute the whole notebook and save it as an object
          with testbook(notebook_path, execute=[0], timeout=600) as tb:
  ~~~
  
  But what if we want to execute the **entire notebook besides the first cell ???**
  Currently it has to be as follow:
  ~~~python
      def test_notebook(self):
          NUM_OF_CELLS = 'some number'
          notebook_path = "fuse_examples/tutorials/hello_world/hello_world.ipynb"
          # Execute the whole notebook and save it as an object
          with testbook(notebook_path, execute=range(1, NUM_OF_CELLS + 1), timeout=600) as tb:
  ~~~
  
  But A MUCH CLEANER APPROACH would be:
  ~~~python
      def test_notebook(self):
          notebook_path = "fuse_examples/tutorials/hello_world/hello_world.ipynb"
          # Execute the whole notebook and save it as an object
          with testbook(notebook_path, skip=[0], timeout=600) as tb:
  ~~~
  
  