Return a standard template value along with a type field

[MacroPower]

Right now, grafana can return template values of either null, string, or string[].

To make deserialization more simple, values should always be represented as string[].

e.g. null = [], string = [""], string[] = ["", "", ...]

Then append a "type" field so that users may still distinguish between different selection options. (e.g. single-select, multi-select).

[MacroPower]

This has been implemented as of #13