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.

[rschili]

@mathieu-fournier this can be closed, right?