Support registry meta-data fields in manifest

[LKedward]

Now that we have fpm-registry I think we should add official support for various meta-data fields in the fpm.toml manifest including description, keywords, categories, and homepage.

Looking to cargo as an example, it requires the following fields before a crate can be published.

Checking and validating the fields is a job for the fpm-registry CI, this issue is just to enable support within fortran fpm.

[awvwgk]

It would require a schema definition or at least any kind of documentation what is allowed in a package manifest, description is actually missing in the PACKAGING.md, which I took as reference for the allowed keys.

Also the two available implementation follow very different strategies. The boostrap implementation ignores everything except the needed keys, while the Fortran implementation does not allow anything except the keys hardcoded in the executable.

[awvwgk]

I think adding all those keywords to the toplevel might overload the package file a bit, having additional sections for meta information would probably be best, e.g. conda-build is using about. We could support an extra section as well, which is not governed by any schema and allows to test new sections or keys.

[awvwgk]

Done with #171.