Add support for stripping whitespace

[nathan-muir]

(After #321 changes are merged into release-2.4)

Other HTML templating languages automatically strip whitespace that includes newlines, eg:

<div>
    <span>A</span> <span>B</span>
</div>

Would be converted to

<div><span>A</span> <span>B</span></span>

This PR does something similar, controlled via an extra template attribute, <template name="..." whitespace="strip">...</template>.

Whitespace between two elements that does not contain a newline, is left untouched:

<div><span>test</span>        <span>space</span></div>