Reference collection item content as strings

[damiani]

This PR is inspired by this question, and adds a __toString method to CollectionItem, to return the contents of a collection item—parsed Markdown, for instance—whenever the item is referenced as a string.

This would allow referencing a specific Markdown collection item in a Blade template like this:

@foreach ($notes as $note)
    {!! $note !!}
@endforeach

...instead of needing to reference $note->getContent().

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This PR includes some changes to path handling, replacing realpath and getRealPath which don't play nicely with the virtual file system used in our tests.

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I'm working simultaneously on addressing the lack of support for getContent() in Blade files (#172) which will also be affected by this change.