[tex4ht] Internalizing "custom links" feature

[Mayuresh]

I find t4ht to be an incredibly useful tool to produce quality htmls that
honour latex constructs to a great extent.

I find the "custom links" as explained in the following document to be a
key feature without which the tool would have been of little use to my
requirement.

http://folk.uio.no/simek/document/html/documentse8.html#x18-260008.2

To use this feature currently one requires inputting above contents to
one's document.

It would be nice to include it as a latex package so that it is inherently
available without having to copy above file and input it into latex code.

Mayuresh.