Version fpm.toml files

[certik]

Just like cmake, we should add an optional toml section with the minimal fpm version required. So when you use some new fpm feature, such as #854, you set the required fpm version appropriately. If the same project gets build with an older fpm version, it will refuse to build it, saying you have to use a newer fpm version.

Some packages, like toml-f might require to work with older versions of fpm, which means that they cannot use many newer features (for a while). Some other packages are ok with requiring a newer fpm version, in which case they can use newer features.