Added fluent stop interface

[jesstelford]

Added a function as the return value to res.startTime for stopping the timer:

app.use((req, res, next) => {
  const stopFileTimer = res.startTime('file', "File IO metric")
  setTimeout(() => {
    stopFileTimer();
  }, 100)
  next()
})

[coveralls]

Coverage remained the same at 100.0% when pulling 2d7d97e on jesstelford:fluent into 9805311 on yosuke-furukawa:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 2d7d97e on jesstelford:fluent into 9805311 on yosuke-furukawa:master.