Add ManifestAppenderTransformer to support sealing packages in jars.

[chrisr3]

Java's manifest attributes are implemented using a HashMap, which makes them useless when the attributes must be written in a particular order. E.g. for sealing packages in jars:

Name: com/example/
Sealed: true

Implement a Transformer which can append tags to a manifest while preserving their order.

Note: This is a WIP - my transformer's current UI is based on Groovy's Tuple2. However, I'm sure that Groovy can support something better.

The transformer's current API is:

shadowJar {
    transform(ManifestAppenderTransformer) {
        append('Name', 'first/sealed/package/')
        append('Sealed', true)
        append('Name', 'second/sealed/package/')
        append('Sealed', true)
    }
}

[chrisr3]

BTW, can this plugin be installed into a local repository? I would like to test it properly on an actual project 😉.