feat: add metadata for beancount postings

[Triple-Z]

Add metadata for generated beancount postings, like the following example.

- 2021-05-20 * "苏宁易购官方旗舰店" "【礼遇价】三星128g内存卡microSD存储卡tf卡行车记录仪卡switch监控摄像头"  ; 支出-交易成功-数码电器
+ 2021-05-20 * "苏宁易购官方旗舰店" "【礼遇价】三星128g内存卡microSD存储卡tf卡行车记录仪卡switch监控摄像头"
    Expenses:Electronics 87.90 CNY
    Assets:Bank:CN:CMB-9876:Savings -87.90 CNY
+       source: "支付宝"
+       payTime: "2021-05-20 12:34"
+       category: "数码电器"
+       status: "交易成功"
+       txStatus: "支出"
+       note: "支出-交易成功-数码电器"

We can also move the comment Note to metadata.

[gaocegege]

SGTM

[Triple-Z]

The transaction's metadatas are shown on Fava.

Schedule this feature to v1.1.0?

/cc @gaocegege

[gaocegege]

SGTM