Tokens&comments updates

Made token & comment format more compatible with Mozilla-style node locations (as used elsewhere in code) and with Esprima's token & comment formats.

This enforces conformance between locations in different types of generic objects (tokens, comments and nodes) and simplifies usage with other tools like Estraverse / Escodegen.

Also, allowed passing arrays instead of functions in `onToken` and `onComment` so comment attachment was slightly simplified (please take look at updated example in README).
It doesn't take much space and anyway any engine will inline this when compiling. While `isArray(options.onToken)` is more readable than `options.onToken && options.onToken.push`.