Support reading/writing environment variables using $env

Implemented support for:

    $env:userprofile
    $env:MyVariable = "MyValue"
Support getting child items of Env: drive.

The following commands now return all environment variables:

    dir env:
    dir env:\
    Get-ChildItem env:
    Get-ChildItem env:\
Support individual environment variables from Env: drive.

The following commands now return an environment variable:

    dir env:userprofile
    dir env:\userprofile
    Get-ChildItem env:userprofile
    Get-ChildItem env:\userprofile
Environment variables can now be selected after changing to Env drive.

Support the following:

    cd env:
    dir userprofile
Merge pull request #222 from mrward/environment-provider

Fixed some parts of the Environment provider