Install Visual Studio Extension

Is your feature request related to a problem? Please describe.
I would like to have my VS extensions installed when I receive my Dev Box

Describe the solution you'd like
As a user, I should be able to:

1. specify a list of extensions (extension version optional) that ought to be installed on my Dev Box, using a list of packageNames from the VS marketplace

setupTasks:

 - task: install-vs-extension
    inputs:
      extension: "packageName@version"

For v1, we will NOT support path to json exports of extensions

 - task: install-vs-extension
    inputs:
      extensions: "/path/to/vsixt"