Added and documented the capability for shlex to handle lexical-level
inclusion and a stack of input sources.  Also, the input stream member
is now documented, and the constructor takes an optional source-filename.
The class provides facilities to generate error messages that track
file and line number.

[GvR: I changed the __main__ code so that it actually stops at EOF, as
Eric surely intended -- however it returned '' instead of the None he
was testing for.]