Added collapseMultipleWhitespaces option

[eliascotto]

Added an option collapseMultipleWhitespaces to replace series of multiple whitespaces with a single one, if true. Otherwise do that only for leading and trailing.

See #361

[martincizek]

With the option turned on, the conversion would become broken. There can be arbitrary sequences of whitespace anywhere in the original HTML. Even though you trim leading and trailing WS runs, the whitespace inside of the text can introduce e.g. paragraphs or indented code blocks that are not present in the original HTML.

Also, mixing ASCII and non-ASCII whitespace differs between HTML and Markdown - see the specs and it would have consequences in escaping rules.

What Turndown tries to do, is to convert the whitespace to the best semantic equivalent. Preserving the insignificant white space from HTML syntax is not completely possible by design. And even though there can be some partial preserving some whitespace, I currently think it's not worth it.

You can check the complexity of dealing with significant whitespace translation, which is currently done for:

• preformatted inline code - Preformatted inline code #318, PR Preformatted inline code #319
• dealing with mixed unicode and ascii whitespace - PR Correct Unicode whitespace collapsing and improve flanking white space #315

I suggest to better describe the need of preserving insignificant WS in the related ticket. Based on that, we should decide whether to rework or close this PR. Thanks.