Start the default browser, when starting the `mkdocs serve` command? about mkdocs

[oprypin]

I'll implement this 👍

[fralau]

See also @pawamoy's comment:

If it can detect that the URL is already open in a tab, then that would be best.

[oprypin]

OK let's say the URL is already open in a tab, what should it do then? Just not open the browser? I think that's worse than opening the browser even if the tab will be duplicated. Unfortunately there's no implementation for "focus an existing tab if possible".

[fralau]

It makes a sense: a duplicated tab should not be a big deal.

[oprypin]

one of the problems I met was with async: the time it might take for the page to load on the browser, versus the time it takes for the server to start.

Hmm for me it worked reliably. Could you try my pull request and see if you run into such issues with it?

[pawamoy]

I think that's worse than opening the browser even if the tab will be duplicated.

Yeah that's what other tools do anyway. We can discard my comment about detecting the tab.