Apple - Safe to charge macbook with USB Type A to USB Type C cable?

Yes, and it is safe. I did charge my 15" MBP from various sources - power banks and adapters (USB Port of a Tesla Model S was my favorite), and it can take around 13W. Just make sure that your adapter supports 10-15W, 5W is quite slow. Unfortunately the port does not support Quick Charge.

When idling, the system consumes around 6W, charging will be impossible while using with something more cpu/gpu intensive.

And if you decide to cheat and try to connect a second charger, it will not combine the power, but instead choose the one with higher power rating.

TL;DR: It's possible. It's safe (It'll cause 100% use on your adapter, might heat it up). Perfect for emergency use or when weight is important. Not useful if you plan to do extensive work.

I'm able to charge my MacBook Pro with my Xiaomi power bank 10000mAh with QuickCharge 3 and it works perfectly! :)