Widget for Life Expectancy

We're missing one widget which is for Life Expectancy.

Tasks

[x] add widget life-expectancy.json in https://github.com/datahq/city-indicators/tree/master/widgets
[x] write Vega specification where we compare life expectancy in London versus UK (male and female) - see graph on https://data.london.gov.uk/
- [x] use vega online tool here - https://vega.github.io/editor/#/examples/vega/line-chart but refer to our example here - https://github.com/datahq/city-indicators/blob/master/widgets/housing.json#L14-L192 - but keep in mind that you need to put "values" array in online editor