Fix invalid date

Take 2018-09-24 for instance, it is a holiday in Taiwan so there is no currency record on that date. However, an error would occur on 2018-10-01 when the app computes the currency change in last week because it can't find the corresponding value. This commit makes the app search for the last valid date for computation.

No Crawling Every time

Data will be crawled and parsed only when this app was launched more than 30 minutes ago.

First launch

Release Date: July 30, 2017

1. 以互動式圖表呈現近三個月的匯率資料
2. 以互動式圖呈現外幣存款牌告利率
3. 比較當日與前一週的即期賣出價格
4. 如果手上已經持有外幣，可以計算現在脫手的投資報酬率