Handle tag bodies split across multiple lines

This fixes the following:

```lisp
plump> (parse "<derp id=\"urrr\">some text</derp>")
#<root {100308F4C3}>
plump> (serialize *)
<derp id="urrr">some text</derp>
nil
plump> (parse "<derp
id=\"urrr\">some text</derp>")
#<root {10030FA033}>
plump> (serialize *)
id=&amp;quot;urrr&amp;quot;&gt;some text
nil
```

The changes are not very pretty, and are the minimum of effort on my part. They could probably be shortened to something like `(whitespace? char)` but that's something for another day.