Easier way to apply a plugin from rc.pl file

[plicease]

This seems to currently be the "correct" way:

preload sub {
  my($meta) = @_;
  $meta->apply_plugin('Fetch::HostAllowList', allow_hosts => [qw( github.com ftp.gnu.org )])
};

but we should also support something like this:

plugin 'Fetch::HostAllowList', allow_hosts => [qw( github.com ftp.gnu.org )];

like you an from an alienfile.

[plicease]

Should update the documentation for plugins Fetch::HostAllowList and Fetch::BlockList when done.

[plicease]

Maybe:

preload_plugin $name, @args;
postload_plugin $name, @args;

[plicease]

349 does this, but we shouldn't close this until the two plugins have updated documentation.

[plicease]

updated the plugins.