Add parsing of math operations in quantity formatter. For input fields.

[mathieu-fournier]

Add the ability to parse simple math operations in quantity formatter.

This will be usefull to do simple calculation inside input fields. Ex: "10ft + 2" "10ft + 6in" "10ft - 1m" "10ft * 2" "10 / 2" (no units, defaults to the active unit system)

[mathieu-fournier]

FYI : I'll clone the repo, implement it and do a PR.