Feature: metalsmith.run() getter

The proposal is to add a parameterless signature to metalsmith.run() so that it functions much like metalsmith.metadata() and metalsmith.ignore().

The method would return a new (read-only) object of type Metalsmith.Run (in TS-land). The object would hold info like duration,start,end, error and would be particularly useful for logging statistics in watch mode.The affordance added by this feature somewhat overlaps with #392