Add support for alias expressions in data targets

[birdflyer-lszo]

Grafana allows setting an alias expression in a data target so that it gets identified nicely in the resulting graph. Target did not yet allow setting such an alias.

Add a simple alias field to the Target class so that the selected data can be aliased with an arbitrary expression.

[JamesGibo]

I wondered if it would be better to combine alias and legendFormat as they serve the same purpose, just for different data sources?
Or just include some docstrings for them?
See this for info on the difference for different data sources (https://community.grafana.com/t/how-to-override-metric-name/4388)

[birdflyer-lszo]

You are right. If they are the same, there is little need for a separate property. I will kill this PR (I am using a different Target now anyway).

Will likely open one or the other PR in the future as I stumble across things i find missing :) The library is very helpful by the way!

[JaSei]

Please do this. I use SqlTarget for influx (to be able to hide) and legendFormat doesn't work as alias

or it will be enough add

alias = self.legendFormat