Allow for stdin input to rt shell action.

[marbud0]

This makes it possible to more easily script bulk operations for RT by allowing to read from STDIN instead of the tty when STDIN is NOT connected to a tty.

For example, this now works: rt ls -t user -i | sed 's/user/show user/' | rt and is aproximately two times faster than rt ls -t user -i | while read a;do rt show $a;done