Only add newlines after <pre>/<textarea>/<listing> if necessary.

The HTML5 serialization spec at
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-end.html#serializing-html-fragments
states that:

"If current node is a pre, textarea, or listing element, and the first
child node of the element, if any, is a Text node **whose character data
has as its first character** a U+000A LINE FEED (LF) character, then
append a U+000A LINE FEED (LF) character."

Currently domino always appends a newline after pre/textarea/listing.
This patch fixes domino to match the WHATWG spec, only adding a newline
where it is necessary to maintain the proper value of the contents.
(Since an initial newline is silently discarded when it follows a
pre/textarea/listing.)

Closes gh #16.