"banner" property

[azproduction]

refs #167

We need a way to define custom banner for bundle. Default value will be:
// This file was automatically generated from ...

It also possible to use lmd licence as banner.

var read = require('fs').readFileSync;

var banner = '/*!\n  * ' + read(require.resolve('lmd/LICENCE')).replace(/\n/g, '\n  * ') + '\n  */';

module.exports = {
    banner: banner,
    modules: {
        ...
    }
};

It also possible to use some variables __dirname and __filename in banner template.

• default value of "banner" is // This file was automatically generated from ...
• it should not be printed if it is a non-string
• possible to use variables in it __dirname, __filename
• builder should warn if banner contains JavaScript (not just a comment block)
• bundles should inherit "banner" value from parent config
• source map should consider banner lines

[leonidborisenko]

Thanks!