Headers in config file for URI-style http ammo

[direvius]

Add Headers config parameter for URI-style http ammo.

Now we can specify headers in ammo file, but there are cases when we want to use same ammo file with different headers. It would be convenient not to regenerate whole file.

See also: #60 #52

[Irisha]

https://github.com/NinaCarrot/pandora/commit/0a89257473ec24b88cddb44fd717c6ebfbe4d3e1
https://github.com/NinaCarrot/pandora/commit/efd153fee6e67ebac4b306fa2cfb692c85f1ef48

We use pandora with this patch.

[nettorta]

#121

[nettorta]

done via https://github.com/yandex/pandora/pull/122/files

• https://github.com/yandex/pandora/wiki/PR-%23122:-%D1%80%D0%B5%D0%B0%D0%BB%D0%B8%D0%B7%D0%B0%D1%86%D0%B8%D1%8F-%D0%BF%D0%B5%D1%80%D0%B5%D0%BE%D0%BF%D1%80%D0%B5%D0%B4%D0%B5%D0%BB%D0%B5%D0%BD%D0%B8%D1%8F-%D1%85%D0%B5%D0%B4%D0%B5%D1%80%D0%BE%D0%B2-%D0%B8-%D0%B5%D0%B5-%D0%BE%D0%BF%D1%82%D0%B8%D0%BC%D0%B8%D0%B7%D0%B0%D1%86%D0%B8%D1%8F.md