Added options object that allows the user turn off the output file hash,...

... and also include a version string in the filename.

Tests pass.

  ✓ should rev files
  ✓ should not rev files if hash is false
  ✓ should rev and version if version set
  ✓ should not rev but add version if version set and hash is false
  ✓ should build a rev manifest file
  ✓ should allow naming the manifest file
  ✓ should append to an existing rev manifest file
  ✓ should respect directories
  ✓ should store the hashes for later

  9 passing (17ms)
But that's not gulpy. One should put logic *in* the gulp file.