Added command-line options --ask and --save (and --version)

Option --ask asks for a positive 'y' or 'yes' response before uploading.
Option --save skips uploading entirely and saves locally.

As this adds several new options, the entire option-parsing
process was rewritten to use OptionParser.

For completeness, support for --version was added as well.
This change also moved the version number out of the .gemspec,
into "lib/showterm/version.rb".