Added new optional PREFIX parameter to define a tag prefix

[L-Sypniewski]

Added a possibility to define an optional PREFIX parameter to be added before x.y.z version number.
Removed WITH_V parameter - omitting PREFIX results in the same behaviour as setting WITH_V to false in a previous versions.

Closes #72

[tautschnig]

@L-Sypniewski Was there a particular reason for closing this PR?

[L-Sypniewski]

Hello, @tautschnig
To be honest it was mistake of mine while I was cleaning up branches.
Nevertheless, I'm working on some other feature (bumping up version by taking into account messages from all commits between the last tag and the one specified as a parameter) so I'll make a new pull request including changes form this one - I'll do it probably within an hour.

[L-Sypniewski]

@tautschnig The changes have been moved to #102