Support babel options that are functions (eg. `getModuleId`)

[Marcin Szczepanski]

Currently we pass the Babel options from the master to the workers via a stringified JSON object, this means we can't pass callback functions such as getModuleId.

We need a solution to be able to pass code as options - probably via some sort of slightly more convoluted require mechanism.

[Marcin Szczepanski]

Fixed in v0.2.3