feat: add poll effect type

[zhanba]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• 添加新的effect 类型 poll，用来处理轮询逻辑
• add new effect type poll

effects: {
    pollAdd: [
       function*(_, { put }) {
           yield put({ type: 'add', payload: 1 });
       },
       { type: 'poll', delay: 1000 },
    ],
},

// start polling
dispatch({ type: 'count/pollAdd-start' });

// stop polling
dispatch({ type: 'count/pollAdd-stop' })

[coveralls]

Coverage increased (+0.4%) to 87.681% when pulling f88198b on zhanba:feat/add-poll-effect into 2b9f315 on dvajs:master.