Strip only new lines when separating content from metadata

When parsing items and separating content from metadata, Nanoc strips the content: https://github.com/nanoc/nanoc/blob/3.7.1/lib/nanoc/data_sources/filesystem.rb#L271

This is bogus and prevents the following Markdown from being correctly compiled:

    ----
    title: I start with a code block
    ----
        this is a code block
        this is a code block

Calling `#strip` removes the leading spaces and breaks the code block.
I like to have a blank line between the metadata section and the content, like this:

```
---
title: "Hello"
---

content goes here
```

What about calling `.sub(/\A[\r\n]{,1}/, '') on the content (instead of `.strip` originally)? That would allow the empty line between metadata and content and it’d still be stripped (it’d be the only thing that is stripped).