Add --version info

[rouson]

I just installed fpm using the current master branch. It would be nice to be able to check the fpm version using something like fpm --version and/or fpm -v. Doing so with my just-installed copy gives

± fpm --version
Invalid option `--version'

Usage: fpm COMMAND [--release] [--args ARGS]

[LKedward]

Hi @rouson, it looks like you are referring to the bootstrap version. This functionality is available in the latest Fortran implementation - this is the version that is currently under continued development.

[rouson]

@LKedward @everythingfunctional I'm replacing CMake with fpm in the dag project and would like to update the list of prerequisites in our README.md, where it would be nice to write something like "fpm version x.y.z or later". Even without the fpm --version functionality mentioned in this issue, it would be great to create and tag releases for downstream documentation purposes.