Support biber comments

[MiWeiss]

Extracted from #133. We may want to support the following biber-type comments:

@ARTICLE{bla,
    author={Fartsy},
    %title={Title},
}

I see two options to handle this:

1. A middleware to filter out fields starting with %
2. Adapting the splitter with a custom flag to ignore lines starting with % or whitespace followed by %

In either case, for within-entry comments, I don't think we should create a Comment block in the library.

Option one is more in line with v2 architecture, and it prevents from flags on the splitter, but inputs like this will fail to parse if %-comments are only handled in a middleware.

@ARTICLE{bla,
    %somecomment
    author={Fartsy},
    title={Title},
}