WCAG2.0-compliant luminosity contrast for <code>

#ff4500 has luminosity contrast of 3.4:1. Need at least 4.5:1. Use #dc3b00.
WCAG2.0-compliant contrast on colour for <code>

#DC3B00 provides contrast of 4.5:1 on white. But to provide a little flexibility, e.g. in the case that some <code> is in a <th> with background #eee, a colour of #C83500 provides 4.56:1 contrast, while still standing out as red text.
Allow references that are missing href.
Support Previous Version on NOTEs

As per a discussion on spec-prod, NOTEs can have Previous Versions, so we need to support output of that header too.
Allow permalink on div with an h* element
Test to ensure permalinks are on h* within div
Merge pull request #319 from halindrome/patch-2

Support Previous Version on NOTEs
Merge pull request #310 from tobie/fix-refs

Allow references that are missing href.
Merge pull request #307 from jasonkiss/develop

More accessible luminosity contrast on colour for <code>
Merge pull request #320 from halindrome/patch-3

Adding permalinks to h* within div
v3.2.16
Don't break NOTES that have no previous version
v3.2.17