make_vsn/0 to integrate with the world around us;

`make_vsn/0` allows people to generate semantic version
numbering scheme whenever and wherever needed just by
calling the `rebar_vsn_plugin:make_vsn/0`.
This is useful especially when you implement a custom
rebar plugin that manipulates `.app` files in a way
incompatible with `rebar_vsn_plugin`.

The original motivation is to bring semver support
for ChicagoBoss-powered web applications.