Update `tuneup' functionality:
    
        - Previous to this commmit, a regular expression was defined to
          set the file extensions supported as pattern. Then, one list of
          files was built using both the action value and the file
          extension regular expression pattern. Later, when processing the
          list of files, backend-specific functionalities were exported
          and unset for each file in the list based on the file extension.
    
        - In this commit, the supported file extensions are no longer
          defined as a regular expression pattern. Instead, we use a
          simple list of values (e.g., `TUNEUP_EXTENSIONS="svg xhtml
          sh"') and loop through it to create one list of files to process
          for each supported extension individually. Then, if the list of
          files is not empty, we export backend-specific functionalities
          and then loop through the list of files to process to apply
          backend-specific maintaince tasks to each file in the list,
          individually.  Once all files in the list has been processed,
          backend-specific functionalities are unset and the next
          supported extension is evaluated in the same way described here.
    
          This commit is much more optimized than previous one. Specially
          for environmnets where many different files need to be processed
          in just one run of `centos-art.sh' script. In this commit, we've
          organized the list of files to process by extension, in order to
          export/unset only the backend-specific functionalities needed
          for extension-specific processing. Nice, doesn't it? :)