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/

Locate the command implementations and browser-opening logic for `ionic serve`, `ionic docs`, `ionic link`, and `ionic signup`; start by comparing how `--browser` is defined and used. Done means every listed browser-opening command accepts `--browser`, supported commands also accept `--browser-args`, and the values are passed to the browser program.

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