Get file-specific version history

## Expected Behavior

I get old versions of the project that were implicitly created via file saving.

## Actual Behavior

I get snapshots of the project that were explicitly created via publishing.

## Steps to Reproduce the Problem

1. Create a project.
1. Modify some file in it and save.
1. Look at File -> See Version History and observe there are multiple versions.
1. `clasp versions` doesn't show these versions.

## Specifications

- Node version (`node -v`): `v8.11.1`
- Version (`clasp -v`): `2.0.1`
- OS (Mac/Linux/Windows): Windows

## Fix

There seems to be no way to actually grab old file versions, and `versions` misleadingly suggests this is possible. Could this functionality be added?