Added options.gitRange

Fixes issue #3: Allow 'after' and 'before' to be git tags

### Usage

If `options.gitRange.after` is defined, `options.before` and `options.after` will be ignored.

#### options.gitRange.after
Type: `string`
Default value: `undefined`

The commit SHA1 value or tag where the log will start at.

#### options.gitRange.before
Type: `string`
Default value: `HEAD`

The commit SHA1 value or tag where the log will end at.

Example:
```
grunt.initConfig({
  changelog: {
    sample: {
      options: {
        gitRange: {
          after: "commit SHA / tag",
          before: "commit SHA / tag"
        }
      }
    }
  },
})

```