allow fetching current data, and passing in old data to be merged with c...

...urrent data when generating reports

The idea is to able to support saving the current data set to a json file, like so:
```
data = JSON.generate Coverband::Reporter.get_current_scov_data
File.write("blah.json", data)
```

Then later on, pass it in to the html reporter:
```
data = JSON.parse(File.read("blah.json"))
Coverband::Reporter.report :additional_scov_data => [data]
```