[Feature] -D / -P switch

Adds two new switches:

-D<key=value>
-P<platform> (alias for -Dplatform=<platform>)

and one new $st method:

  $st->getParams()

This allows the user, via the command-line, to inject parameters into the story and/or the template, which are always available via the new $st->getParams() call.

We're adding this to allow multi-platform support in stories and story templates (ie running the same story on a local VagrantVm, or a PhysicalHost, or on EC2 with no code changes).