Guard against code execution with malicious download options

```ruby
options = { :git => 'example.com', :branch => '; say hi' }
target_path = '/tmp/testing-stuff'
downloader = Pod::Downloader.for_target(target_path, options)
downloader.download
```

Note, this needs a lot of testing. All I know is the unit tests work, I haven't manually tried all of this because I don't even know how to test some of these providers. We need to build a full test suite which could test ever option each provider offers.