Allow setting notes in commit_options from session data

[dometto]

Resolves #1766

• Renames the #commit_message method to the more accurate #commit_options (since it already returns author data in addition to the message)
• Adds a :notes key to the hash returned by #commit_options, the value of which is sourced from the session
  • the relevant session key (gollum.notes) can be set by a piece of middleware, analogous to the way this works for author info

Possible use cases are e.g. getting the client IP and using this info in the :post_commit Hook. In the future, we could provide a feature to actually store the contents of the :notes field in a git note. For now, this only passes the info on to the Committer object. gollum/gollum-lib#431 ensures the note is stored in the Committer object, where it can be accessed from the Hook.