resolves #470 allow uri to be used in inline image

- don't mangle imagesdir value if its a uri
add compatibility for handling absolute and uri image targets
Merge pull request #477 from mojavelinux/inline-image-with-url

resolves #470 allow uri to be used in inline image
resolves #478 make callout bullets non-selectable

- enforce that callout bullet is at end of the line
- consume line comment character immediately preceding callout number
- change layout of callouts in html output