Allow custom version numbers

I've changed the bump functionality so that one can specify custom (but valid) version numbers.
This is very handy under heavy development, where you need to add patch, pre-release, and build identifiers (as in http://semver.org rule 12).

```
semver-sync -b 3.0.0-alpha1
[OK] Version number was updated to 3.0.1-alpha1 in package.json, component.json, mypackage.js.
semver-sync -b 3.0.0-beta2
[OK] Version number was updated to 3.0.1-beta2 in package.json, component.json, mypackage.js.
semver-sync -b 3.0.0-rc1
[OK] Version number was updated to 3.0.1-rc1 in package.json, component.json, mypackage.js.
````