Create a version defined by user

[ericfer]

It could exist a command line to create a version defined by user.
For example:
$ stepup version
v2.0.3.rc10+2

$ stepup version create v10.0.0.0beta
Created version v10.0.0.0beta

$ stepup version
v10.0.0.0beta

The purpose would be to provide an initial use of the gem in a late stage of a project or even a way to manage an strategic decision of the project owner to increase version levels at once