More plasticity rules

[chaoming0625]

Please:

• Check for duplicate requests.
• Describe your goal, and if possible provide a code snippet with a motivating example.

It is exciting that brainpy supports flexible customization of CPU and GPU operators (see #518). I think the next important step is to provide more plasticity rules, like STDP, triplet STDP, reward-based STDP, and many others, in which the operator customization interface will help a lot.

• STDP
• dopamine-modulated STDP
• Triplet STDP synapse
• ......