Add a way to open a new browser when using `rio run`

[mad-moo]

Having a new browser window open each time when rio run is executed can be quite annoying, especially for projects that necessitate frequent restarts. Add a way to prevent this from happening.

I see two options for this:

1. Add a flag. This is probably the way to go, but is a bit annoying because it needs specifying each time
2. Add a configuration option to rio.toml. The issue with this one is that this preference really depends on the developer, so adding it to the project-wide rio.toml could lead to conflicts