Url list option

adding an option --url_list FILENAME to check files (urls) specified in file FILENAME where each line of the file contains a file (URL). If the option is set checker validates both the files in this file and the ones specified in command-line arguments. That allows start vnu.jar only once to check any numbers of pages.

Also adding in cli-help:

--url_list FILENAME

Specifies a filename with files (URLs) to be checked. Each line of the 
file contains a file (URL). If the option is set checker validates both 
the files in this file and the ones specified in command-line arguments.

default: [unset; vnu validates only files (URLs) specified after all the 
options or in standard input]