Accept pre-release labels

[fdaciuk]

We should accept the labels alpha, beta and rc labels (using this precedence).

Examples:

cargo v alpha

This command will add a -alpha.1 next to patch version or increase the number after -alpha..

• If current version is 1.0.0, the result must be 1.0.0-alpha.1.
• If current version is 1.0.0-alpha.2, the result must be 1.0.0-alpha.3;
• If current version is 1.0.0-beta.1, the command above should result in an error: you can't decrease a version.

The same can be done with beta and rc labels.