Nicer folding

This is a slightly half-baked attempt to address #12.

I've incorporated @vovkkk's code and improved the handling of Ctrl/Cmd+Enter, but it's still problematic in the following cases:

* When word wrap is on and the fold marker is on its own line.
```
# Heading 1
[...]
Et cetera
```

* There is only one heading and the rest of the document is folded.
```
# Heading 1[...]
```