[Enhancement] Add ability to track depreciation / residual value

[vpmv]

A lot of household assets (e.g. appliances, cars) don't keep their original value. It'd be a nice addition to add a field with the current value, and maybe even do some percentage-wise automated depreciation.

A somewhat related addition would be a "price per period". For example: I own my car for ten years, how much did it cost me per year (e.g. $25.000 / 10 = $2.500/y). Together with the residual value, you can make a better estimate in whether it's appropriate to replace the asset.

This issue relates to #98