Support code fences within comments #46

Often it is useful to run some code without having it display in the rendered markdown. Two such cases:

1. Running some setup code. Yes, fixtures are a workaround but sometimes each file needs its own setup code so it doesn't make sense having a bunch of fixtures in conftest.py.
2. Running assertions to test our code. Often, these assertions should be hidden from those reading the documentation.

I tried placing the following in my documentation but it isn't being detected?

<!--
```python continuation
assert something == something_else
```
-->

Any workarounds or fixes?