Support for mathematical calculations

[martynhaigh]

Adding support for basic calculations in the size and offset fields would improve readablilty:

{
    "keylines": [
        {
            "offset": 56,
            "from": "TOP"
        }, {
            "offset": 56 + 24,
            "from": "TOP"
        }, {
            "offset": 56 + (2 * 24),
            "from": "TOP"
        }
    ]
}