add 'px' complement to getSizeFromBreakpoint

[woo-cie]

Hi. First, I'm grateful for the very nice module. This PR implements #30.

• Add completion of px to number in getSizeFromBreakpoint
• Add number to DefaultBreakpoints type
• Enable to convert number as px in pxToEmOrRem
• Add generics constraint to pxToEr and pxToRem