Add the ability to switch between user, path and machine context for configuring path variable

[andy1547]

This one is fairly self explanatory, just involves adding a context parameter using a validation set, defaults to machine if nothing is specified.

[jasonmcboyd]

Just have to keep in mind that the DSC local configuration manager (LCM) runs under the System account, not the account of the user that called Start-DscConfiguration. So it is the System account's 'Path' variable that will be updated. There is the the 'PsDscRunAsCredential' parameter (I don't know what version of PowerShell that parameter was introduced in) that does allow for resources to be configured under a different account on a case by case basis. In fact, you can probably assume that if the 'PsDscContext' variable is present (it will be if a value was passed for the PsDscRunAsCredential parameter) inside the function that the function should target the user environment rather than the machine environment obviating the need for a switch.