Add support for float font sizes

[SNBSLibs]

I know you don't want new font formats. But font size is different, and the integer-only support makes this library a bit inflexible.

On my SSD1306 blue OLED display, font of size 1 is really small (56 characters take up just a half of the display), while font of size 2 is a bit large (56 characters don't fit in there). I suggest adding support for float font sizes to make them more flexible.

I've modified the library to implement it. My code currently does the job by just increasing the gaps between letters, but it looks much better than size 1 or 2.

Should I issue a PR?