Add support for --trailer

[Gorfeywog]

Similar to #129 it would be nice if I could call git absorb --trailer key=value. Of course the trailers would get lost after rebasing to fixup, but it could be nice if I wanted to push the fixup! commits for review first.

[tummychow]

i would accept a pr for this under the same constraints as #129 - fixed string, specified on the cli only, applied to all commits created by git-absorb with no ability to pick which ones