add delay option

Sometimes the response is too fast (e.g. cache) so delay is good solution to avoid loading spinner if that's shows only for few milliseconds.

Here is my util fot that:

```javascript
import { createActionHelpers } from 'vuex-loading'

import { delay } from 'lodash/function'

export function createDelayedLoading (options, wait = 1) {
let actionHelpers = createActionHelpers(options)
let { startLoading: startLoadingFn, endLoading: endLoadingFn } = actionHelpers
if (wait) {
let delayed = null
actionHelpers = {
startLoading: (dispatcher, loader, callback) => {
delayed = delay(() => {
startLoadingFn(dispatcher, loader, callback)
}, wait)
},
endLoading: (dispatcher, loader) => {
clearTimeout(delayed)
endLoadingFn(dispatcher, loader)
}
}
}
return actionHelpers
}

```

What do you think about adding something similar to the project?