walk: opts.onFile is not inclided in promissory chain

[matutter]

The onfile callback, if it returns a promise, doesn't get included in the promissory chain and isn't cat by the exception handler for profs.walk.

[matutter]

Supposedly this code doesn't work

var opts = {
  onFile = file => new Promise(resolve =>{
    setTimeout(() => resolve(first = true), 3000)
  }) 
}

profs.walk('.', opts).then( () => assert(first, 'walk.onFile callback still going') )