org-e-html: Improve handling of images, headlines and inlinetasks
1. Handle inlining of remote images
2. Proper handling of "alt" attribute for regular imgages and dvipnged
fragments.
3. Support for Inlinetasks
4. Headlines are "transcoded" in three places:
- org-e-html-headline (`org-e-html-format-headline')
- org-e-html-inlinetask
- org-e-html-toc (`org-e-html-format-toc-headline')
These are now conveniently wrapped and cleared through
'org-e-html-format-headline--wrap'. The regular and toc headlines
formatters mentioned afore take a superset of arguments allowed by
`org-e-html-format-headline-function'. Augmenting the signature of
the `org-e-html-format-headline-function' is worth considering.