Trailer support

[Uncommon]

Trailers are metadata stored at the end of a commit message, usually in the format <tag>: <value>. For example, the official Git repository uses trailers such as "Signed-off-by" and "Reported-by".

Things that could be done with trailers:

• [x] Display them specially in the commit info display, such as highlighting them or showing them at the top as if they were part of the actual commit metadata.
• [ ] Allow columns to be shown in the history list for trailer tags.
• [ ] Add trailer tags to the list of things that can be searched.
• [ ] Facilitate adding trailer tags when entering a commit message.
• [ ] Recognized trailers defined in config, such as trailer.<token>.key. (Unfortunately such config settings don't automatically come down with a cloned repository.)
• [ ] UI for defining trailer tokens.