Position-specific tab-expansion

(as opposed to always replacing with 4 spaces)

Ref: https://github.com/vfmd/vfmd-spec/issues/2
Don't mention tabs for code-block syntax

This reverts commit f87f3ea7fcc8c3059e3b0e089fa4d5c4270163e6.
Syntax guide: Add a note at the start regarding tab stops and expanding tabs
Reorg "Encoding" and "Tabs" into a new subheading
Merge pull request #4 from roop/expand_tabs

Tabs should be expanded based on their position in the line

Resolves vfmd/vfmd-spec#2