resolves #16, #24; closes #20

* For #16, use Cross-Origin XHR to check if there's `history.go(-1)` to
check whether it's a valid page in 全國法規資料庫. `permissions` is added in
`manifest.json`.
* For #20, auto re-assign `document.body.innerHTML` to "查無資料" (in
Chinese) for 404 pages at 立法院法律系統. It can be improved further, but
that's enough for now.
* #24 is resolved slightly differently from the algorithm talked about
in the issue page.
* A tab could be a disabled link, therefore user can right-click and
open it in a new browser tab/window.