[PATCH] [yaml2obj] Add new command line option `-docnum`

[Simon Atanasyan]

Thanks for review.

Initially I planed to implement something like `-docname` option. But I could not figure out how to name YAML documents. We can use tags for that but it is a good idea to reserve the tags for output file format specification (ELF | COFF). Maybe solution is to support some sort of "complex" tag format like `!ELF|<doc label>`.

Closed by commit rL209967.

http://reviews.llvm.org/D3901