Method format refactored to make it more maintenable, replacing the
switch by a function factory, that returns the appropiated function
given the character (d, i , f, j, s).

Also, performance when formatting an string that contains several
consecutive % symbols is improved. The test:
`const numSamples = 10000000;
const strPercents = '%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%s%%%%%%%%%%%%%%%%%i%%%%%%%%%%%%%%%%%%%%%%%%%%';
var s;
console.time('Percents');
for (let i = 0; i < numSamples; i++) {
  s = util.format(strPercents, 'test', 12);
}
console.timeEnd('Percents');`

Original time:   28399.708ms
After refactor:  23763.788ms
Improved: 16%

Improved performance

Fix lint error: Infix operators must be spaced

Improving performance.

- Correct whitespace
- Static parts of if before unstatic.