Add option to prevent overriding releases.

[Gerrit91]

Hi there!

Not sure if you're interested in this PR but at least I wanted to offer this to you.

I like this action a lot but it's important to me that already published charts will not be overwritten once published (references #13).

Unfortunately, my skills in Bash are limited but I tried my best and it looks like it's working already. :sweat_smile:

Regards!

[nshores]

This would be very useful to me, and i'd like to see this merged in if possible.

[Gerrit91]

You can use my fork, it's still working sine the days I've raised this pull request.