Add setting for portable browser

[elfia]

Hi,
This add-on is quite handy but I did not find a way to add portable browser in the add-on's browsers list. It would be nice to have this option for the following use cases:

1. If you don't want to install a browser locally but want to test against it through the IDE
2. If you are testing against several versions of a browser and you want to be able to run the test with a fixed browser version from the IDE directly.
   I am on a macOS Mojave 10.14.1
   Visual Studio code version is: 1.40.1
   Testcafe version is: 1.7.0
   Testcafe Test Runner add-on is: 2.0.0