Implement an "invisible" cell tag, or metadata attribute

There are some cells that are required for the .ipynb file to work in development, but shouldn't be shown to the end user. For example, there are some cells that need to have a resources/ directory added to the path variable for imports to work:

```python
# This needs a "hidden" attribute in cell metadata.
import os, sys
cwd = os.getcwd()
resources_path = os.path.join(cwd, 'resources')
if resources_path not in sys.path:
sys.path.append(resources_path)
```

This kind of cell needs either an `invisible` tag or cell metadata attribute, so it can be removed when the notebook is converted to html. This is distinct from `hidden`, which the user can toggle. For example, cell output can be toggled by the user.