added file copying and the option to specify additional file types to preprocess (e.g., .js, .css, etc.)

Now, all files contained within the sourc directory are moved to the
destination directory. Those not preprocessed are merely copied.

Additionally, one can now specify additional file types by there
extension to be preprocessed, for example:

php prehp.php -css -js processor.php /usr/source usr/dest
updated documentation to reflect new file-type options