Netlify-cms: feature: Add commit message templates

[BerkeleyTrue]

- Do you want to request a feature or report a bug?
feature
- What is the current behavior?
Currently the user has no control of how the application writes commit messages.
- If the current behavior is a bug, please provide the steps to reproduce.

- What is the expected behavior?
It be awesome for users to provide templates through the config. This would allow users to stick with popular commit message formats like conventional-changelog-angular.

- Please mention your node.js, and operating system version.

[tech4him1]

@BerkeleyTrue That's an awesome idea, thanks for filing this!

[knpwrs]

This is something that I am potentially interested in implementing. @BerkeleyTrue, were you thinking something in the admin interface or something in the configuration? My idea was for something in the configuration, either globally or perhaps at the collection level, that would allow the commit message to say something like User ${user} updated page ${page}.

It looks like right now the commit message is hard-coded between "Create" and "Update". I could imagine something like this in the configuration to support different commit messages without having to build out an interface:

commit:
  type:
    # Override default uppercase verbage for conventional changelog
    update: update
    create: create
  message: "${type}: ${file} by ${user}"

My main interest is tying each individual commit to a user, so my use-case has no need for a UI.

[erquhart]

@knpwrs this looks solid as a general approach, feel free to take a stab.

I think the simplest configuration option would be to just accept a separate message for each type and not try to combine them. This provides max configurability and less room for confusion about what the config keys do:

commit_messages:
  update: "update: {{file}} by {{user}}"
  create: "create: {{file}} by {{user}}"