Add support for a opts.requireByIndex option for numerically indexed requires

Add support for a new requireByIndex option, which uses node-detective
to find the require statements and replaces the original argument with
a numeric index, allowing faster and more compressible lookups; this
goes some way towards a proof-of-concept for
https://github.com/substack/node-browserify/issues/296 , although only
for literal-based requires