RFE: allow setting a bodhi update message for `build`

[sanjayankur31]

When using fbrnch build -F to build and push updates for all Fedora branches, one can set the type of update, but one cannot currently provide an update message/comment. Would an option that allows this be useful to add?

A particular usecase I'm looking at is to update all the branches without it requiring user-interaction---build for each branch and if the build succeeds, push and update and move to the next branch.