Charge MacBook 12" while connected with Google's USB-C to DisplayPort cable
Take a look at these two multiport adapters from Apple:


USB-C VGA Multiport Adapter (USB-C to USB-C/VGA/USB)
USB-C Digital AV Multiport Adapter (USB-C to USB-C/HDMI/USB)


There's no technical reason why third parties won't come out with adapters since all of this is standard. USB C charging, video formats, the connector type are all non-proprietary. Only time will tell if businesses invest in making these products, but Apple's two adapters work very well for the use case you describe.

Also keep an eye out for cables and monitors that send the required USB charging current over USB-C most higher quality displays should soon be able to charge your computer like Apple's previous generation displays now that the proprietary charge connection is gone from the MacBook (and others).