feat: add skipCi option

[flaval]

This PR add the ability to plugin's user to specify the way he wants to pass the "skip ci" information.
By default, the [skip ci] keyword is passed to the commit message, but this behavior is not always the required one. Worse, it is hard fixed to the default commit message, and the plugin's user have to copy/paste the default commit message omitting this keyword if he doesn't want it.
This option permit to specify message value to keep the default behavior, pushOption value to use the git "push-option" functionnality (a warning is raised in README about git version compliance) or false to simply remove the keyword in default message.

It answsers to a (already closed) issue #171.

Like I said in this issue, I'm not a really good english speaker/writer, so forgive my comment and please review my README updates. 😅

Another thing, it is my very first contribution/PR on github, I hope I correctly follow the process 🙂

[flaval]

Hello, is there someone here ? @pvdlg are you still here ?
Have I a chance for my PR to be reviewed ?
Thanks

[riconnon]

I'm a big fan of this... hopefully it can be reviewed.

[flaval]

Thanks, but I'm starting to lose hope

[rhtenhove]

@travi is this something that you'd be interested in? It looks like a great way to clean up the release message.

[tommy31]

Big fan of this feature. Any update ?

[gfaugere-safti]

@travi If you are interested I could probably rework this in a git push --push-option feature, which I think would be more inline with the "generic git" philosophy of this module?

[gfaugere-safti]

Anyone from @semantic-release interested in helping moving this feature forward?

[genofire]

Wow, why is this not merged ?

@travi @gr2m @UziTech