Added `sourcePriorCommit`

Added the token `sourcePriorCommit` that ref prior commit hash e.g. `HEAD~1` to make a messages like this possible:

```js
Built %sourceName% from commit %sourceCommit% on branch %sourceBranch%
github comparision link: https://github.com/kevinawoo/grunt-build-control/compare/%sourcePriorCommit%...%sourceCommit%
```