Add support for em TextStyles #787

Use case

It is common practice to have larger letter spacing depend on the size of the text.

Flutter only supports setting a letter spacing in terms of pixels.
This makes it much more challenging to re-use themes. A letter-spacing must be set for every font size.

Proposal

Add a property called letterSpacingScale to TextStyler which will used to set the letterSpacing.

This idea is found in CSS where it is defined using em.

(Maybe we should add rem too! This would be really neat for Flutter Web)