Add a 'skip' option and document the term_width parameter to the constructor

This pull request fixes a couple of minor issues I've had.

First, I've documented the 'term_width' argument to the constructor. I've had issues ssh'ing into a remote machine and then finding the term_width defaulting to 50 with a warning that it couldn't determine the terminal size. Digging through the source code helped me find this undocumented function.

Next, I sometimes find myself working on machines where Term::ProgressBar doesn't work due to issues with the underlying modules (can't recall the details right now), or I just want to easily silence the module with a switch. So I added the 'silent' option to the constructor. If passed a true value, then Term::ProgressBar does nothing, but you don't need to change any other code.