Reindent numbered lists in CONTRIBUTING.md

Markdown not only requires multi-paragraph list items to be indented at
the same level, but explicitly requires either 1 tab or 4 spaces
([Markdown Syntax#list](http://daringfireball.net/projects/markdown/syntax#list)).

Reindent 3 -> 4 spaces, as the 3-space indent was throwing off
Marked.app (the second paragraph in the list item was being interpreted
as at the top level). (I believe Marked.app uses MultiMarkdown out of
the box.)
Merge pull request #474 from sharplet/contributing-ocd

Reindent numbered lists in CONTRIBUTING.md
Revert pull request #427.

Reverts c811d87, 090a4ab, 253322a, and e86cc03, part of pull
request #427, in favor of resolving issue #473.

This removes the `CONFIG_START` and `CONFIG_END` macros in the hopes of
adding them again, after further testing, at a later date.
Revert "Revert pull request #427."

This reverts commit 503f7e0638bc4cdcd9994242e6aa95303baa04c6.
Merge branch 'revert-revert' into config-fixes