Prop label size should be allow for any unit, such as the more standard "rem" unit

[jakebesworth]

em as a unit of measurement is a less modern "font size" unit of measurement. Many frameworks (such as the 4.0.0 bootstrap) try to use "rem" (root em) instead, which does a proportional size compared to a single root size (for use in scaling the UI of an app, usually in @media queries). Other applications might use em, or pt, or any other measurement.

I think an apt change would be to:

1. Change the default to "0.7rem". Right now, 0.7em is quite large for any app that does both mobile and desktop.
2. Allow the labelSize prop to accept a unit attached such as "12px", "12pt", "12em" (or a labelSizeUnit prop)

As with the other issue, might try and tackle this myself, making issue for reference.

Also, I'm not sure if this is a bug, but "labelSize" PR seemed to only edit files in dist/ not the files in src/ but I'm not sure if that is intentional or not.