Banner/License comment support

[andvgal]

I am adding this issue as a backlog as it is not really critical, but it may get a blocker for users obeying licensing rules (e.g. GPL).

There are many cases, when license or banner has to be included in result file (e.g. by license requirement).
Yes, it is possible to achieve now, but it is not very convenient.

1. The simple way is to add a "banner" option similar browserify. It solves the problem partially.
2. However, if sources from more than one module are included, it may be required to add several banners in result file header comment.
   My proposal is to add:
   a. "requireBanner" option to enforce checks that banner exists for every bundled module
   b. "bannerMap" option for { require_name : banner_text } pairs
3. To make it more advanced and flexible, the "banner" and "bannerMap" options should get pre-populated with "banner" field from the corresponding module's package.json file, if present. It will ensure that always the correct banner text is included and no copy-paste is needed.