Make getAltText optional

I wanted to translate the navigation texts in french.
I used the "formatter" prop, however, for some reason the getAltText function is mandatory.

**Steps to reproduce the behavior:**

```
const formatters = {
getNextTitle: () => { return "Suivant (flèche droite)"; }, // Next (right arrow)
getPrevTitle: () => { return "Précédent (flèche gauche)"; } // Previous (left arrow)
}

```

**Expected behavior:**

Supply formatters with an empty object should not modify the behaviour.
Better document getAltText

**Actual behavior:**

1) Crash : getAltText is mandatory
2) getAltText doesn't take a CommonProps parameter as it's written in the doc (https://jossmac.github.io/react-images/#/)
It receives this :
`{index: 3, data: { a ViewType item }}`

Thanks