Added usage information
Added main function to process argument(s)

The main function will treat either the current directory (in case of no
arguments) or the given file or directory as the argument to process.
Top level function created

The process() function will deal with the input based on whether it is a
valid file or a directory. If neither, the script will terminate with an
error.
Handling added for dealing with directories

When a directory is given, the script launches recursively till it finds
only files. It will avoid entering symbolic links so as to not enter
into any infinite loops (which in practice are finite because the system
will not resolve the loop beyond a certain number of times).
Note that replay gain will be recorded in the tags
Normalization subroutine added

The subroutine will only add on regular files with a .flac extension.
The file will be silently re-encoded in place. Then, the file will be
analyzed for replay gain and the values will be written to its tags.

The subroutine will avoid symbolically linked files.
Write debug message when avoiding symlinks
Script is now executable
Add some comments explaining steps
Implement -h option for help

Calling for help will only print the help message and exit.
Version 1.0