Support launching browser in WSL Ubuntu 18.04

[jiasli]

Fix #332

The code is migrated from Azure CLI:

https://github.com/Azure/azure-cli/blob/3bff786b9e337aa35bb4cc64a09e5600a35663f8/src/azure-cli-core/azure/cli/core/util.py#L642-L648

    if is_wsl():   # windows 10 linux subsystem
        try:
            # https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_powershell_exe
            # Ampersand (&) should be quoted
            return subprocess.call(['powershell.exe', '-Command', 'Start-Process "{}"'.format(url)])
        except OSError:  # WSL might be too old  # FileNotFoundError introduced in Python 3
            pass

_browse now tries webbrowser.open first. If that fails and WSL is detected, launch browser with powershell.exe.

As for why powershell.exe is preferred over cmd.exe, it is discussed in Azure/azure-cli#12851.

[jiasli]

www-browser internally also uses powershell.exe

https://github.com/wslutilities/wslu/blob/221e85457c01e92db675cc7b24a49dfcdfacc6a9/src/wslu-header#L156

	"$(interop_prefix)$(sysdrive_prefix)"/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -NoProfile -NonInteractive –ExecutionPolicy Bypass -Command "[Console]::OutputEncoding = [System.Text.Encoding]::UTF8; [Console]::InputEncoding = [System.Text.Encoding]::GetEncoding($cp); $@"

But we can keep it simple.