Add all commit messages that are part of push to message field

[kenmorse]

This excellent GitHub Action would be even better if it could (perhaps optionally) output all the commit messages that are part of a push, rather than just the most recent commit message.

For example, if an author makes three separate commits, then does a push, only the latest commit will be reported by action-slack; there will be no mention of the first two commit messages.

So without this feature, we see:

message
commit msg #3

And with this new feature, we would see:

message
commit msg #1
commit msg #2
commit msg #3

[8398a7]

Thank you for your suggestion.
It seems that there are cases where displaying all messages can be very long.
We need to think about what specifications are good.