SCRIPTABLE : load script from local file

FEATURE REQUEST

For `SCRIPTABLE` strategy:

Currently the script code needs to be specified inline in the XML like this:

```xml

SCRIPT

GROOVY
<![CDATA[
... groovy script here ...
]]>

```

It would be beneficial if the script could be loaded from a file rather than being specified inline. This would allow unit testing on the script code. I'm thinking a file located in the same location as `jgitver.config.xml` itself. Because a script can be malicious we may not want to be too relaxed as to where the script might come from. Haven't thought it through.

In the absolute ideal situation it could be even better if the script file could be specified by a Maven GAV. But let's take some baby steps first. :-)