Add rule MD005 - inconsistent bullet indentation
Document the Markdownlint::Doc class

This class provides several helper methods that are heavily used in rule
writing. Document appropriately.
Add rule MD006 - unordered lists at beginning of line
Add non-recursive option to find_type and find_type_elements

This simplifies MD006, and will simplify future rules.
Add rules MD007 and MD008 for nested list indent

These rules cause multiple rules to fire for some lines, so the test script
has been modified to allow multiple expected rule tags on a line.
Add rules MD009/MD010 to check for whitespace issues
Add some more tags for rules
Add rule MD011 - Reversed link syntax

Also abstract out the 'errors are lines that match this regular expression'
into a Doc method.