Added min and max font size; Modified the default font size to 13 px

Added minimum and maximum font size for customisation, which are not required. I also changed the minimum font size to 13 points which is the suggested minimum font size in iOS' guideline.

If neither `minSize` nor `maxSize` are set, the default of 13 px is used. If only `minSize` is set, the default `maxSize` is twice that value, which was the original behaviour.