users should be able to change the default behavior for issues with no label

[FrancescoCioria]

requirements

this issue is the outcome of a discussion on this PR: #96 (comment)

Right now, issues without known labels, are always treated as "features".

Some users may prefer to ignore issues without known labels or treat them as "breaking" or "bugs" instead of "features".

We should improve smooth-release to enable them to do so.

specs

Improve the config with two new options:

• one to decide the label for feature issues (def "feature"?)
• the other option as an enum to decide how to treat issues with no known labels and treat it by default as "feature-issues".

NOTE: by treating issues without know labels as "feature-issues" by default, this new feature will not be breaking 🌮

misc

{optional: other useful info}