Attach symlink property to Vinyl objects when passed through symlink method

[phated]

As per @erikkemperman's comments in #246, we should be attaching the .symlink property in other scenarios (other than just src(..., { resolveSymlinks: false })). The most obvious of these is when the file passes through the symlink method and a symlink is created (I think this can be done in symlink's prepare stream to reduce some path history hackiness).

[erikkemperman]

That's exactly what I'd done earlier but never got around to submitting: attach symlink property before changing file.path in prepare, and then use that instead of file.history[-2].

[phated]

Feel free to submit it if you get a chance. I'm not tackling this currently.

[erikkemperman]

Will do, as soon as I find some time!

[erikkemperman]

@phated I think the file.symlink property should be absolute, like file.path, even for relative links. Does that sound right?

[erikkemperman]

Hm on second thought I'm not sure: src just attaches the result of fs.readLink for symlinks, which might be relative.

[erikkemperman]

Thanks for review and merge! Sorry about the bogus refs above, guess I shouldn't include issue ids until the commit is final.