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. 
Will release a stable when I get back from vacation next week.

In composer.json:

> @@ -25,7 +25,8 @@
>          "php": ">=5.3.3",
>          "symfony/console": "~2.0",
>          "symfony/yaml": "~2.0",
> -        "symfony/process": "~2.0"
> +        "symfony/process": "~2.0",
> +        "vierbergenlars/php-semver": "2.*@dev"

ideal would be a stable release ..

ping @vierbergenlars <https://github.com/vierbergenlars>

—
Reply to this email directly or view it on GitHub
<https://github.com/liip/RMT/pull/73/files#r14878614>.