Teach Parser to resolve relative Image Links

[theory]

Follow-up to #38: Resolve image src attributes that point to a relative file, as in Apache Age, where some images display properly on GitHub but not on PGXN. Perhaps update them to point to the files where they're unpacked for browsing.

In other words, for <img src="/img/age-01.png" />, detect that img/age-01.png is relative to the project directory and update the link to <img src="/src/apacheage/apacheage-1.4.0/img/age-01.png" />