Patch phile v1.x

I'm currently bumping my blog to Phile-v1.x. Unfortunately, this was the only plugin that doesn't have support for Phile-v1.x...

*was*?

Yes, *was* indeed, I saved you again :wink: . I've forked this project and make it work for the latest version.

There's no big deal though, it involves moving around some files and class/namespace renaming. I've also updated the README with installation instructions.

On another note, I recommend you to tag your current repo **before** merging this pull request, so people using the older version can still checkout for it.

Also: I've assumed your vendor name is `frodox`, so that's why I've put it in the namespace `Phile\Plugin\Frodox\PhileCustomPostPreview` and used the `frodox` directoryname in the installation instruction. I don't know if you're using [composer](https://getcomposer.org/) and what your actual vendor name is, but you can always change that after the merge of course. (I would recommend to make this plugin available through composer, because it's easier to maintain updates than downloading zips from all the plugin repos :) )


You can read it all about [here](https://github.com/PhileCMS/Phile/wiki/%5BDEVELOPER%5D-Plugins).