*_SLUG envs are not truncated

[TomaszG]

*_SLUG environment variables are commonly used as URLs or docker image tags. To make it happen without problems, a lot of systems also shorten them, e.g. GitLab's max length of SLUG is 63 bytes (https://docs.gitlab.com/ee/ci/variables/predefined_variables.html). Unfortunately, SLUG variables returned by this plugin are not shortened. Maybe it would be worth either:

1. have a max length of SLUG predefined, or
2. have a configuration option to define max SLUG length, or
3. add a shortened version of SLUG env vars.

[FranzDiebold]

Hi @TomaszG,

thanks for the issue and the idea!

I truncated all *_SLUG variables to 63 characters and published a new version v2.6.0.