Add option to render whitespace

[Sub6Resources]

Some sources remove <br /> tags in favor of using the newline characters as line breaks when rendering. A parameter needs to be added (perhaps renderWhitespace) that defaults to false.

[Atulin]

I added flutter_html to my app today, and it renders code with \r\n as new line perfectly fine. Although it could be because each paragraph is in its own <p></p>

[Sub6Resources]

Yeah, a <p></p> tag would render newlines because it is a block element. This feature is more for rendering html that uses newlines instead of <br /> to display a line break (WordPress, for example replaces all <br /> tags with a newline)

[Sub6Resources]

This is fixed in 0.5.6 (f1e5607)

(I decided to use renderNewline instead of renderWhitespace since spaces are still treated normally).

[clotodex]

For anyone having trouble finding this in newer versions:
https://github.com/Sub6Resources/flutter_html/wiki/Migration-Guides#rendernewlines-deprecated