Provide --browser and --browser-args for all commands that open the browser

`ionic serve` has a `--browser` option, but the following commands (which also open the browser) do not:

* `ionic docs`
* `ionic link` (for third-party git setup)
* `ionic signup`

Additionally, for each command that accepts a `--browser` option, accept a `--browser-args` option to pass additional options to the browser program. e.g. for Chrome, these are the available options: https://peter.sh/experiments/chromium-command-line-switches/

找出 `ionic serve`、`ionic docs`、`ionic link` 和 `ionic signup` 的命令實作與瀏覽器開啟邏輯；先比較 `--browser` 的定義與使用方式。完成的條件是：列出的每個瀏覽器開啟命令都接受 `--browser`，支援的命令也接受 `--browser-args`，並且這些值會傳遞給瀏覽器程式。

由索引模型根據 Issue 內容生成。