version in the case of HEAD

Hello,

When the plugin reaches the HEAD, it outputs zilla->version, and $release->version in all other cases. But $release->version are in reality tags, and /if/ the dzil release is going to do a Git::Tag, it will then obey to the config:

```ini
[Git::Tag]          ; tag repo with custom tag
tag_format = v%v
```

Let's go to the problem. For example, suppose we go from version 0.13 to 0.14, tagged v0.13 and v0.14:

Your plugin will generate this Changes file:

```
0.13
 - something
v0.12
 - before
```

which is commited under tag v0.13. The next iteration will produce:

```
0.14
 - something else
v0.13
 - something
v0.12
 - before
```

so the diff on the commit will be:

```diff
1,3c1
< 0.14
<  - something else
< v0.13
---
> 0.13
```
You see the problem.

IMHO the best solution would be to inherit from Git::Tag and recuperate its tag_format. In case you want a standalone solution, this could to be coded in your module. I did that in a lazy way, because it fits MY setup: coding inside the module, with /only/ the %v format string modifier. While Git::Tag allows more.

So please do not consider pulling this request as is, but, instead, please consider this pull request as a former proof of concept of the problem.
What is the best solution, /if/ you dare to tackle this problem, I do not know....

Thanks,

Regards, Jean-Damien.