adding config flags to control all items mentioned in PRIVACY.md

closes #3241
- in config.js, the `privacy` attribute holds all privacy-related flags
- `privacy.tinfoil: true` disables everything (equivalent to setting all flags to false)
- added helper function to core/server/config/index.js to checking privacy flags
- added helper function to core/server/config/index.js to show warning about deprecated items
- updating config.example.js with commented-out flags and description


Once this is merged in, http://support.ghost.org/config still needs to be updated with the new config options
If I didn't already know how `tinfoil` worked, I'm not sure it would be clear after reading the description and seeing the example configuration block below.

What if we change it to something like:

```
// Setting the `tinfoil` option to `true` disables ALL 3rd party services.  The `tinfoil` option
// takes precedence over all other options in the `privacy` section.

//privacy: {
//    tinfoil: true,
//    updateCheck: false,
//    googleFonts: false,
//    gravatar: false,
//    rpcPing: false
//},