Add -CountVariable parameter

[peombwa]

This PR closes #152 by proposing the following changes:

• Ensures all $count=true is always in lowercase. MS Graph doesn't support Camel cased count values.
• Adds a runtime -CountVariable parameter to all cmdlets that return a collection. This will be used by customers to get back a deserialized value of @odata.count returned in the response body. By default, this variable will be stored in the global scope.
• Adds -CV alias for -CountVariable.
• Makes -Count parameter private i.e. it won't be exposed to customers.
• Ensures $count=true is always set with -CountVariable is present.

Proposed use

[georgend]

@peombwa Ready to Merge?