insert 2 spaces instead of tabs

I like 2 spaces for code indent, but it use 4 spaces by default.

so I wish macdown could add a preference to set count of spaces.

Start by locating MacDown's indentation preference handling and the code that inserts indentation when editing. Add a configurable two-space option while preserving the existing default, then verify that new indentation follows the selected preference.

Written by the indexing model from the issue text.