Add support for tab-literals

I'd like to generate tab-delimited files with `plaintextgen`. The current work-around is to split-extract some whitespace (tabs are 'imported' as 4 spaces), and adding a property macro that returns a tab literal "\t". The exported file then contains a tab literal.

Unfortunately, there seems to be no support for adding literal tabs otherwise, so I will try to make a pull-request for:

* showing literal tab characters (via a special arrow glyph?)
* editor support (via intention) to insert a literal tab
* importing unstructured text with literal tabs in the body (not at the start of the file)