Add option to preserve trailing whitespace

[charith-elastic]

I am using my own set of templates to produce Asciidoc documents. Removing all trailing whitespace from the rendered document causes problems with Asciidoc rendering so this PR adds an option to preserve them.

It also adds a safeIdentifier template function to strip special characters from anchor identifiers. This is because some tools don't accept identifiers such as type.domain.com/v1

[ahmetb]

Yeah we can totally do this. The only reason we remove trailing spaces on each line is to prevent github markdown from misrendering it. I left one comment.