Implemented decorator pattern with notebook_loader

[rohitsanj]

Implements #5

1. Created a class called notebook_loader which encapsulates the contextmanager and decorator pattern of setting up tests.

• contextmanager pattern:

from testbook import notebook_loader

def test_foo():
    with notebook_loader('/path/to/notebook.ipynb', prerun=[0, 1]) as notebook:
        notebook.execute(['test1', 'test2'])
        assert notebook.cell_output_text('test2') == 'foobar'

• decorator pattern:

from testbook import notebook_loader

@notebook_loader('/path/to/notebook.ipynb', prerun=[0, 1])
def test_foo(notebook):
    notebook.execute(['test1', 'test2'])
    assert notebook.cell_output_text('test2') == 'foobar'

2. Added prerun argument to notebook_loader

[MSeal]

We should maybe make nbclient change the name to cleanup_kernel since it's needed to manage kernel states

[MSeal]

We should maybe make nbclient have a startup_kernel since it's needed to manage kernel states