Add banner option

[martypdx]

Allows inclusion of a banner at the top of concatenated files:

var register = components
    .transform('import', {
        assign: function ( filename, importedAs ) {
            return `Ractive.components['${filename}'] = ${importedAs};`;
        }
    })
    .transform('concat', {
        banner: 'import Ractive from \'ractive\';\n\n',
        dest: 'components.js',
        writeSourcemap: false
    });