Add feature to ignore certain mods

[PureSpider]

There are some packs that have client side only mods that crash the server when used on the server side.
For this, it would be useful to have an option to ignore certain mods.

[dries007]

I'll add this, but please tell mod authors to include the correct FMLSide in the manifest for example like this:

jar { 
    manifest {
        attributes 'ModSide': 'CLIENT'
    }
}

or in the @Mod annotation in more recent Forge version (there is a clientSideOnly property you can set to True.