Allow modules in pytest header to be customized

With this, affiliated packages can do (in ``conftest.py``):

```
PYTEST_HEADER_MODULES['scikit-image'] = 'skimage'
```
I think you're thinking of:
```python
__import__('package.submodule')
```
only returns the package, not the submodule, bizarrely.  This can be worked around though with:
```python
mod = __import__('package.submodule', fromlist=[''])
```
which is incredibly obscure but works.  This is also why `importlib.import_module` exists now :) (albeit only in Python 3.3+)