add support for relative widths to imgcap

This adds support for relative widths to the `imgcap` and `imgcaption`
(block version) tag. The syntax is `{% imgcap [right|left|center] <path or url>
[<width>[%] height[%]] [<title> [<alt>]] %}`, and similar for
`imgcaption`
* If `width` and `height` are expressed as percents, the outmost span
  gets a `style='width:xx%;'` tag, and the inner img and caption span
  are sized to 100%.
* If `width` and `height` are expressed as integers, the inner img tag
  gets the specified width and height, the inner caption span gets a
  `style='width:(xx-10)px;'` tag to account for the margins, and the
  outer span gets no specified width so it sizes to fit its contents.

I've tried to make it versatile but haven't included much extra error
checking. Specifically, calling width as relative and height as absolute
(or vice versa) may result in undefined behavior.

Also, the about page it updated to use the new imgcap syntax
add feature captions and alt to post-index
update feature image for archive page