Image attributes

Implemented image attributes as discussed in #261.

The changes to `pandoc-types` can be found in [this pull request](https://github.com/jgm/pandoc-types/pull/14).

All writers except the following have been adjusted:

- Docx
- ODT,
- OpenDocument
- Custom
- RTF (the generated RTF files don't work well for me anyhow, I should probably open a separate issue for that)

Would be great if someone familiar with those formats could adjust the writers.

The README and markdown reader have been adjusted as well. I chose to avoid renaming an existing extension flag, but maybe it would be better to rename the existing `link_attributes` to `mmd_link_attributes`, and my new `common_link_attributes` to `link_attributes` (or if you know of a better name than common, the basic syntax is shared by Markdown Extra, Kramdown and now Pandoc).