Applying formatting across lines

Nobody has claimed this yet.

Right now MacDown will apply formatting across line breaks, even though the renderer doesn't apply it that way. For example, in the following text, highlight from two to three and apply bold formatting:

```
two

three
```

Result:

```
**two

three**
```

So no text is bolded at all. (Same thing happens if you format multiple list items at once.

Desired result is:

```
**two**

**three**
```

where both items are bolded.

I think if MacDown watches for line breaks and applies the formatting tags to each line separately, that should work. For list items, you'd want to exclude the list markers, of course.

Reproduce the multiline and multi-list-item formatting examples in MacDown, comparing the current output with the desired per-line formatting. Trace the formatting action from the selected text and verify that line breaks and list markers are handled correctly. Done means each selected line renders with its own formatting tags without wrapping across line breaks.

Written by the indexing model from the issue text.