Implement full metadata extraction to JSON

[LKedward]

I've made a start on this as part of listing fpm packages on fortran-lang.org, so creating an issue to assign.

As discussed in fortran-lang/fpm#34, this is just a script that reads the registry.toml and generates an index.json file containing relevant data from the fpm.toml in each package. I'll then use this JSON file to list the fpm packages on the website.