Also resolve path of src/href attributes.

[mauvm]

This PR makes referencing to local files (including images) work with the basedir option, just like the include/extend commands do.

This is based on two assumptions:

1. Escaped attributes, in this case "src" and "href", always start and end with double quotes
   (I couldn't find a clean way to extract the URL, suggestions are welcome);
2. The basedir option is also used in front end,
   which would make img(src="https://github.com/test/image.png") compile to <img src="https://github.com/base/dir/test/image.png"/>.

Please verify.

[mauvm]

This needs some further examination.