Rename 'edited' hook to more accurate 'reflow'

Also under consideration was `layout`, but that has other meanings
whereas reflow is more specific:
https://github.com/mathquill/mathquill/pull/317#issuecomment-45383191

Just did:
```sh
sed -i '' s/edited/reflow/g $(git grep -l edited)
```
and rewrote the README entry.

Recommend `--word-diff-regex='\w+|\S'` to view diff.