Add option to dynamically specify a list of labels to be added to created PRs

[geekpete]

I see adding labels is possible by using the config file but it would be neat to be able to specify labels at runtime with a parameter.
It might also be good to have two modes to do this, one for replacing the label list (eg only using the provided list) and another for providing additional labels to any already defined in the config file.

[sorenlouv]

This is now mostly possible after #115 has been merged.

You can do

backport --branch 6.0 --upstream elastic/elasticsearch --labels myLabel

Note: this will replace any labels specified in .backportrc.json config file. I can probably add an option like --append-label - or would it be better if --labels appended automatically instead of overwriting?

[sorenlouv]

Closing this. LMK if it needs to be re-opened.