feat: remSize for dynamic REM

[itsezc]

This resolves the issue highlighted on #49 as the prior PR relates to the rootSize of Master units, this PR introduces a new property: remSize therefore avoiding any breaking changes.

remSize accepts a number, that is used to change the mapping from px to rem, i.e. when remSize is set to 10, 10rem units represent 100px, where as before it would've represented 160px.

To also keep the browsers happy, this resets the body fontSize to 16px (or 1 rem) automatically.

Additionally, to keep the IDE and linters happy, I've added an .editorconfig.