Command line option for file name trimming in long paths to prevent long path issues in Windows

Added a command line option -t <len> to set maximum path length. File names in long paths will be truncated to fit the specified maximum length. The recommended parameter is 259. 
Directory names are not affected as trimming folder names would require more changes in different parts of the code. So if file name can't be trimmed to fit the specified length, it is not changed and file will not be downloaded just as it happened before this fix. However, my ~20 courses downloaded fine with "-t 259".