Append to insert

when some body use pytest-pythonpath, `insert` path is more useful than `append`.
Could we change for that?
With the code like this, the list of python paths will be added in reverse order.  So if you have this:

    [pytest]
    python_paths = your/path/apps your/path/libs

Your PYTHONPATH will look like this:

    ['your/path/libs', 'your/path/apps', ...]

Where the `...` is what was already in your path before adding the new paths.  Could you fix this so they are inserted at the beginning in the order they are defined?