Add metadata for licenses

[dominiccooney]

I'm imagining one stumbling block for people who have deployed JavaScript and want to serve BinAST will be preserving licenses that are currently in the JavaScript source.

For example, Babel has a shouldPrintComment hook which by default doesn't strip comments which look like licenses.

[Yoric]

We could add a generic metadata footer (rather than header, for performance reasons) with a (key, value) map.