Add an option to txt to not touch whitespace in pre-formatted strings

[jchavarri]

Sometimes in tyxml apps one receives a pre-formatted string that is rendered with white-space: pre;.

At the moment, the only way to handle this case is by disabling indentation for whole tree altogether. It'd be nice to add an option to txt to honour new lines and other whitespace specifically for a single text element.

[Khady]

Note that the moment the text content is modify is during printing, for example with Format.printf "%a" (Tyxml.Html.pp ~indent:true ()) html. If there is a long line of text, it will be wrapped by format to respect a column limit.