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)
since you do not have any defaul values set, this line can simply changed with `opt = opt || {};`
otherwise it would be nice to document the defaults here like this:
```
var defaultOpts = {hash: true, version: ''};
opt = objectAssign(defaultOpts, opt || {});
```