Converted the extension lists and the color table to be psobjects
so that the user can customize each by either adding extensions or
changing the default colors. Made the psobject names more specific
to better namespace the objects since they needed to be promoted to
global scope.

By making two global objects the user can either customize the colors
of individual file extensions by mutating/adding them individually,
or they can iterate over the preprovided list of extensions and
mutate them in groups.

Example usage in Powershell Profile:

```ps1
$GetChildItemColorTable['Directory'] = "Blue"

ForEach ($Exe in $GetChildItemColorExtensions.ExecutableList) {
    $GetChildItemColorTable[$Exe] = "Green"
}
```