Support riot pre-processors

If you use Riot pre-processors, you can provide config options for riot-jest-transformer to register pre-processors before compiling your tags for your tests. In this case you should use the module format for jest configuration (i.e. exporting your configuration object as a module from jest.config.js). For config scheme, check the readme: https://github.com/tompascall/riot-jest-transformer#setting-up-jest-config-file.