add pre/post-save hooks

- `ContentsManager.pre_save_hook` runs on the path and model with content
- `FileContentsManager.post_save_hook` runs on the filesystem path and model without content

- use pre_save_hook for things like stripping output
- use post_save_hook for things like nbconvert --to python
`--script` triggers post_save hook with nbconvert
update `--script` behavior to use `nbconvert --to script`
docstring
Merge pull request #6896 from minrk/save-hooks

Proposal: add pre/post-save hooks