[Feature request] Support for patch metadata

[shtrom]

It would be useful to extract SCM metadata, such as author or commit message, from the patch header, if present.

[shtrom]

Notes from @calixteman:

It should be easy to do: https://github.com/mozilla/rust-parsepatch/blob/master/src/parser.rs#L324-L331
Just add a line before the while to call a header parser