Added StringArray Function

[pjdufour]

This PR adds a StringArray() []string function to each value. I will make a PR to viper after this is merged, so that the StringArray variables use this new function. This is needed since binding to pflag prevents the use of values that can't be packed/unpacked using CSV. With this function, viper can pull array values directly from pflag maintaining the original values.

[CLAassistant]

All committers have signed the CLA.

[eparis]

You added a new function to every defined type which appears to only have meaning on StringArray and StringSlice. Why? If it isn't part of the Value interface no one can come to rely on it generically, so they would have had to cast to a specific pflag type. And if they cast to a specific pflag type why would they bother to use StringArray() for any other types?

My second question is, what type of string can't be packed/unpacked with a CSV? Can you write some test that shows the problem?

[pjdufour]

The specific CLI command using cobra/viper/pflag that I needed to run was:

go run main.go serve -d '{name:dc_amenities, uri: `"data/" + @z + "-" + @x + "-" + @y + "/dc_amenities.geojsonl"`}'

It works with this PR. I first tried a quick hack in viper using reflection to handle this case; however, it was not feasible without refractoring. You can't type cast to access fields from un-exported structs. All the values are un-exported types. I'm happy to consider other potential ways.

[pjdufour]

Are there any outstanding questions that I can help answer? Thanks for consideration.

[therealmitchconnors]

@pjdufour is this change still of interest to you? I am bumping up against similar limitations, and I'm considering creating something like a SliceValue and ArrayValue interface to export list-specific functions for Viper to consume. Interested in collaborating?