Implement `Notes` element

Fountain has a "Notes" element (definition [here](https://fountain.io/syntax#section-notes)) which styles author's notes in the screenplay, so it's clear these are authors notes and not action.

Someone should implement this!

Acceptance criteria (A/C):

- Add a `Note` variant to the [Line](https://docs.rs/fountain/0.1.12/fountain/data/enum.Line.html) enum
- Add a parser for it to [parse.rs](https://github.com/adamchalmers/fountain-rs/blob/master/fountain/src/parse.rs) and call the parser in the [`block` parser](https://github.com/adamchalmers/fountain-rs/blob/master/fountain/src/parse.rs#L171-L188)
- Add a unit test