Add paragraph detection

[vmenger]

Clinical texts often container paragraphs, it would be nice to have a structured way to detect these (along with their heading). This information can also be picked up by subsequent components, e.g. an entity in a paragraph 'Family History' should be labeled with an appropriate qualifier.

Some inspiration: https://github.com/medspacy/medspacy/tree/master/medspacy/section_detection
Maybe even incorporate this directly?