Allow renaming variables via a new options.rename function

[idpaterson]

This pull request adds a rename option expecting a function that, given the name of a variable in the final output, can return a modified name. As shown in the README example, I wanted to convert names to camel case for easier usability.

If a developer chooses to rename destructively such that two distinct variables could have the same name only the last value is included in the JSON output.