v2: letter-spacing auto px

[porsager]

There's a little inconsistency with letter-spacing and auto-px since letter-spacing allows floats as a unit. Bss can't guess when the intention is to have px auto added or if a fraction was actually desired.

A current workaround is to do letter-spacing: calc(2) but I'd like a better solution for v2, either just exclude the letter-spacing property from the auto-px list, or some other more clever thing :P ?