Move number of lines to a constant
Add command line help flag

Including '-h' or '--help' on the command line will show a help
message.
Add option to always output file headers

By default tail will only output file headers when tailing more than one
file. The '--verbose' option forces tail to output file headers all the
time.
Add option to never output file headers

The '--quiet' option supresses the printing of file headers, regardless
of the '--verbose' option.