Fix excessive <br> tags in lists using break-on-newline extra (issue #394)

[Crozzers]

The break-on-newline extra uses a regexp to replace 0 or more spaces followed by a new line, with a <br /> tag.
This is a problem for <ul>, <ol> and mixed lists, where list items are defined on new lines.
Something like

- Item 1
    - Item 2

Could end up as:

<ul>
<li>Item 1<br />
<ul><br />
<li>Item 2</li><br />
</ul></li>
</ul>

The proposed fix replaces 0 or more spaces followed by a new line, but only if they are not followed by a ul, ol or li opening/closing tag.

[nicholasserra]

Nice thank you