feature: support URL masquerade

[major0]

Summary

Support testing of statically generated websites via a --masquerade option similar to the W3C's checklink tool. This would allow running linkchecker as part of a pre-commit hook when generating static websites and leverage all the fun features of linkchecker.

--masquerade "real-prefix surrogate-prefix"
   Perform a simple string substitution: URIs which begin with the string "real-prefix" are rewritten
   using the "surrogate-prefix" before being dereferenced.  Useful for making a local directory
   masquerade as a remote one.
 
   For example:
      --masquerade "http://example.com/x/y/z/ file:///my/local/dir/"

   If the document being checked contains a link to http://example.com/x/y/z/foo.html, then the
   local file system will be checked for file:///my/local/dir/foo.html.

    --masquerade takes a single argument consisting of two URIs, separated by whitespace.  The
   quote marks are not part of the argument, but one usual way of providing a value with embedded
   whitespace is to enclose it in quotes.