Open should respect `BROWSER` environment variable

[pi0]

Context: https://github.com/nuxt/cli/issues/326

[jpsc]

Looking at possible solutions for this, a few questions:

• Is it worth to add dotenv as a dependency for this or is there a simpler way/idea to approach this?
• If the browser is not available should it return warning and open with default one?

[HummingMind]

+1