File System Jail

The [file system jail](https://github.com/jekyll/jekyll-filesystem-jail) proxies all access to the file system to ensure compliance with certain security requirements. This pull request replaces all calls to the file system with calls to the proxy.

/cc @mastahyeti for his :eyes: 

More work is required on the jail gem, but I've purposely loosely-required the jail gem so it can be updated according to SemVer (minor bump for large-ish changes) without requiring a release of Jekyll.