Add dev-port-timeout opt to dev command

[jrschumacher]

Added support for --dev-port-timeout to solve Server is not listening on port XXXX within some environments. (#1007 #730 #486 #431)

Failing lint job is solved in #1633

Tests were not implemented since I could not find any existing tests covering src/api/dev.ts or src/cli.ts.

./sapper dev -h

  Description
    Start a development server

  Usage
    $ sapper dev [options]

  Options
    //...
    --dev-port            Specify a port for development server
    --dev-port-timeout    Specify a timeout for the dev port watcher  (default 5000)
   //...

---

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[jrschumacher]

Added the duplicate default value to communicate the default value via CLI help menu.

[benmccann]

Thanks, but we won't be adding any new options since we're focused on SvelteKit right now.

[jrschumacher]

@benmccann I get that, but this is a blocker for some, including myself. Without this I cannot use Sapper without running from my fork. I'm happy to change this to hardcode a value >5s.