Make whitespace handling configurable; always remove some space around HORIZONTALLINE

[felixfontein]

Currently all whitespace is kept while parsing. This is potentially problematic, for example in RST or MarkDown output, where whitespace can have semantic (for example indentation, double newlines), and whitespace can result in invalid output (say when processing "B(a\nb)" and converting to RST, you get a newline inside a role argument that shouldn't be there).

As a first step to get rid of these problems, allow to configure how to handle whitespace during parsing. The default is as now (ignore). You can now strip whitespace, or keep newlines (so that line-breaks are kept when converting to RST or MarkDown markup, for example).

I also added code to strip some whitespace (spaces and tabs) around HORIZONTALLINE. These result in whitespace in resulting MarkDown or RST markup that shouldn't be there, and there is no reason to keep these at all.