Support nested compound filters

[Chongyi Zheng]

Per notion reference, the compound filters can be nested. Currently, CompoundFilter is defined as map[FilterOperator][]PropertyFilter, thus only property filters are allowed inside of a compound filter.

It's better to have a Filter interface, and let both CompoundFilter and PropertyFilter to implement the interface.

I can try to do it in the coming days.

[Anatoly Nosov]

Hi, @harryzcy!
Looks like this issue is fixed already in #86.
Should be released soon

[Chongyi Zheng]

Closing as #86 is merged