Revisit extra table in package manifest

[awvwgk]

This might be helpful for plugins that want to store their configuration in the package manifest while avoiding to clash with any actual fpm reserved keywords. All fpm has to do is ignore any entries in a special table (maybe named extra(s), plugin(s), ...).

See #232 for the original proposal.

[certik]

As suggested here: https://fortran-lang.discourse.group/t/joining-the-team/1626/6, we could use this to specify formatting options like:

[extra.lfortran.fmt]
indent = 3

That makes sense, to have all such project specific options in fpm.toml and we can teach fpm about formatting via LFortran (and/or other tools).

[awvwgk]

I opened lfortran!487.