[RFC] Add example pdb checker scenario.

Here's an example behave scenario testing the pdb checker. Running `behave` gives the following output (though with more colour):

```
Feature: pdb checker # features/pdb_checker.feature:1

  Background:   # features/pdb_checker.feature:3

  Scenario: When enabled, the pdb checker raises an error with pdb statements        # features/pdb_checker.feature:7
    Given that I am in a git repository                                              # ../../home/daniel/dev/captainhook/features/steps/git_steps.py:7 0.006s
    And I have installed captainhook                                                 # ../../home/daniel/dev/captainhook/features/steps/install_steps.py:6 0.069s
    When I create a file called "pdb_fail.py" containing a pdb.set_trace() statement # ../../home/daniel/dev/captainhook/features/steps/file_steps.py:4 0.000s
    And I git add "pdb_fail.py"                                                      # ../../home/daniel/dev/captainhook/features/steps/git_steps.py:12 0.006s
    And I attempt to commit                                                          # ../../home/daniel/dev/captainhook/features/steps/git_steps.py:17 0.239s
    Then I see an error                                                              # ../../home/daniel/dev/captainhook/features/steps/output_steps.py:5 0.000s
    And there are uncommitted changes                                                # ../../home/daniel/dev/captainhook/features/steps/git_steps.py:22 0.006s

1 feature passed, 0 failed, 0 skipped
1 scenario passed, 0 failed, 0 skipped
7 steps passed, 0 failed, 0 skipped, 0 undefined
Took 0m0.326s
```