fix(pagination): formatter functions now accept strings as return values

[Lehoczky]

Currently both formatFractionCurrent and formatFractionTotal accept only callbacks which return a number. This makes it impossible write functions like this when using TypeScript (without using any):

pagination: {
  el: '#swiper',
  formatFractionCurrent(number) {
      const formatter = new Intl.NumberFormat('en-US', { ... })
      return formatter.format(number)
  },
},

The above code produces this error: Type '(number: number) => string' is not assignable to type '(number: number) => number'.

Since the values returned by theese functions are directly used inside the DOM7 elements .text() functions (see this and this line) it's safe to change the function signatures.

[nolimits4web]

Merged, thanks!