Work out whether proper representation of tabs is warranted

[jackdewinter]

Function ParserHelper.detabify_string is used in various places to simplify the handling of tabs. Rule Md010 also exists to look for tabs and warn when they show up.

from issuses:

md010 - tabs "converted"
      - tabs in code blocks

Based on experience, not sure if there is any benefit to processing tabs as tabs, instead of the spaces they translate into.

This needs further investigation.