Add ability to prepend comments without mangling sourcemaps

[jsoverson]

There's been a use case that has come up as part of grunt-contrib-uglify (#22 where different banners need to be appended to the minified and unminified files.

This case isn't really covered by the --comments flag unless the banner is appended beforehand to an intermediate file which is cumbersome and souremaps get broken if appended after uglification.

Any chance of getting this option?

[mishoo]

I added it as a parameter to the code generator. If you're using UglifyJS.minify function, you can pass for example:

output: {
    preamble: "// (c) ACME"
}

A newline will be added after the preamble if one is not there already.

[jsoverson]

Thanks @mishoo