add width field to Image

[rwst]

Issue 332 in pandoc jgm/pandoc#332
requires an additional data field that goes with an Image.
Allowed values would be integers between 0 and 100, or maybe 200.
If empty, 100 is default.

[rwst]

https://gist.github.com/1380600

The disadvantage of this patch is that the pandoc source must be patched
in many places. Would it be possible to "overload the constructor" in Haskell
to give a default value for this parameter?

[jgm]

This is now obsolete since we have image attributes.