Allow user to add completion for powershell alias (#1621)

When a user has an alias in powershell, she will need to register that
alias for completion.  To make that possible, we store the completion
logic into a scriptblock variable which can easily be accessed by the
user to register aliases.

For example, if the user defines an alias for `helm`:
   PS> sal h helm
she will need to register the alias like so:
   PS> Register-ArgumentCompleter -CommandName 'h' -ScriptBlock $__helmCompleterBlock

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>