I'm actually in your exact situation, and have decided to do the following:
I got a power bank with two USB outputs. I intend to connect the first output directly into the raspberry Pi, and then connect the second output to a breadboard to power the motors.
USB 1 -> Raspberry pi
USB 2 -> Breadboard -> Motors
However, if my power bank just had one USB output, I would have connected it to the breadboard and powered the pi from there as well (through the 5V pin on the PI, I believe):
Power Bank -> USB -> Breadboard
PS: Remember to connect the grounds
In order to connect the USB to the breadboard, I will either cut open an old USB cable and solder the power wires to the board from directly, or look online for something premade that does this neatly for me.
PPS: I haven't tested this yet, so do it at your own risk :)