Accept versions with a 'v' prefix #57

A common convention I have seen across various projects is to prefix the semantic version string with a v (e.g. v1.2.3). I was wondering if there would be interest in supporting it here. I would believe the scope to be fairly limited (allow users to specify strings prefixed with v when instantiating and make the str of the Version class include a v prefix, if appropriate).

I respect this deviates from the semver standards and would understand if you declined the request. I was unsure how you were balancing strictly following the semver standards versus user convenience in this library.

The development team at my employer uses this library a ton and has adapted the current version to accept v prefixes. We would be willing to share our implementation if there was an appetite for supporting this customization, but are, of course, open to other implementations and suggestions.

Thank you again for all of your work on this project. We have found it very useful and use it almost everywhere.