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. 
I can't find validator function in used semver lib.