Permit routes definition to be excluded from build

[kadamwhite]

If I am consuming this library from an application I control, I may have defined a subset of routes I care about (e.g. in a plugin for which this app is a client); I may have no need of the default posts, media etc. As such, the default bootstrapping logic should likely be gated behind a conditional that can be specified in a webpack build (how process.env.production is used to exclude debugging code, for example) to exclude that module from being included & parsed. This ought to notably slim down the final build.