Support including authors as Co-authored-by

Add a config option, git-together.co-authored, which will switch
git-together from using Signed-off-by to Co-authored-by.

This makes it easier to support mobbing, as all authors are included in
the commit message, and this is supported by GitHub (c.f.
https://help.github.com/en/github/committing-changes-to-your-project/creating-a-commit-with-multiple-authors
). This is mutually exlusive with signing off commits - you have to
choose one or the other. This is mostly because it doesn't make sense to
include a single extra committer as the one who signed it off, but it
also means that, in the event that a commit _must_ be signed off, e.g.,
when committing to the Linux Kernel, you don't lose information.

This is a little fragile, as we are parsing multiple git options, and
potentially reading from stdin, as there are at least six different ways
to create a commit message in Git, four of which we care about, and two
of which we should at least acknowledge, even though we don't modify
them in any way.

This also fixes kejadlen#20 by excluding --signoff from the list of args when
using --amend.

Cargo.toml

@@ -7,3 +7,4 @@ edition = "2018"
 [dependencies]
 error-chain = "0.10"
 git2 = "0.7"
+tempfile = "3.1.0"

src/errors.rs

@@ -1 +1,6 @@
-error_chain! {}
+error_chain! {
+    foreign_links {
+        Fmt(::std::fmt::Error);
+        Io(::std::io::Error);
+    }
+}