Mark release as beta

Fix for #71.

How it work?

* new option is added - ```label```. It have 3 choices (RC, beta, alpha, none). If none is choosen then new version will be without label. 

example workflow:

```
Current version: 0.0.0
type: p
label: a
Next Version: 0.0.1-alpha

Current version: 0.0.1-alpha
type: p
label: a
Next Version: 0.0.1-alpha2

Current version: 0.0.1-alpha2
type: p
label: b
Next Version: 0.0.1-beta

Current version: 0.0.1-beta
type: p
label: n
Next Version: 0.0.1

Current version: 0.0.1
type: p
label: n
Next Version: 0.0.2

Current version: 0.0.b
type: m
label: b
Next Version: 1.0.0-beta

```

Any help with this is welcome. 
that being said .. seems like the end of the if statement could be refactored to no longer contain duplicate code in the else clause