Add Where-Object command.

The Where-Object command currently only supports using a script block.

    1,2,3,4 | Where-Object { $_ -ge 3 }
TEST: Enable RosettaCode tests using Where-Object
Implement Where-Object equals parameter.

The following are now supported:

    Where-Object -Property Name -Value 'MyValue' -eq
    Where-Object name -eq MyValue
Merge pull request #328 from mrward/where-object

Implement Where-Object command