Added custom link command support to the config file.

For example, the following is an example of a standard config file for
Windows with the standard link command explicitly given. 
					
<?xml version="1.0" encoding="UTF-8"?>
<shadow arch="32" os="Windows">
  <system>.</system>
  <import>.</import>
  <link>gcc -x assembler -</link>
</shadow>