[util] added doc test utility

[FrankLeeeee]

This PR added a utility command to test the markdown file.

docer test -p <path/to/markdown>

If you want to test the Python code in your markdown file, you need to provide a command to trigger the test. Do add the following line to the top of your file and replace $command with the actual command. Do note that the markdown will be converted into a Python file. Assuming you have a demo.md file, the test file generated will be demo.py. Therefore, you should use demo.py in your command, e.g. python demo.py.

<!-- doc-test-command: $command  -->

Meanwhile, only code labelled as a Python code block will be considered for testing.

    ```python
    print("hello world")
    ```

Lastly, if you want to skip some code, you just need to add the following annotations to tell docer to discard the wrapped code for testing.

    <!--- doc-test-ignore-start -->
    ```python
    print("hello world")
    ```
    <!--- doc-test-ignore-end -->