Add option to take clean snapshots (instead of manual cleaning)

[bitwiseman]

One of the most common scenarios is needing to take a new snapshot for a test.
Instead of making people file the right resources folder, we should make github.test.takeSnapshot default to snapshotting in a clean directory and then replacing the existing snapshot files. The current behavior could be moved to github.test.takeMergedSnapshot (or some similar name).

Or allow that value of github.test.takeSnapshot to be merge?

The same should be done with github.test.useProxy - it would use only proxy files from a clean directory and we'd add a github.test.useMergedProxy.