Better display for units

[ajkeller34]

SIUnits.jl seemed to handle this pretty well. There's no superscript slash in Unicode, so printing rational exponents using Unicode only will be problematic.

[mlhetland]

I see this is closed, but I thought I wouldn't open a new issue just for this: Might it be a good idea to differentiate between compact and non-compact display? Primarily removing the space in compact display, I guess, so show(1cm) gives you 1 cm but showcompact yields 1cm? (Similar to operator spacing for complex numbers, for example.)