Add an 'Indent' configuration option.

This patch adds an 'Indent' key which can be set in a document's
metadata section in order to configure the number of spaces that
Markdown expects as an indent. If the value isn't set, it
defaults to 4.

Tabs are, of course, accepted regardless.
Merge pull request #204 from mikewest/constant

Add an 'Indent' configuration option.