Enable parsing of feature json columns

[bcodell]

This PR:

• adds a function called parse_column which returns table_alias.column and extracts the column from the activity stream's feature_json column if the column name is not identified as a standard column from the Activity Schema, then applies that function to all columns selected from all primary and appended columns in the dataset macro
• updates an existing test (first_ever_1) to extract json features from the primary and appended activity
• updates example__activity_stream and relevant output__<dataset> csv files so that feature_json values aren't nested in an array and so that feature_json keys use snake case formatting
• updates documentation for the included_columns argument in the Readme

[bcodell]

@tnightengale looking for the following feedback:

• Was not being able to specify a feature_json key as an included column a bug or did this functionality intentionally not exist? It seems like you intended to implement it since you had the json_unpack_key helper function already in place (asking for semantic versioning purposes)
• Does the abstraction I put in place make sense, or are there changes you'd like to see?
• Does this assumption (baked into the code make sense: that if a specified included column isn't identified as one of the standard columns (or project-specific aliases) from the Activity Schema spec, it is assumed to be contained in the feature_json?
• Are the semantic changes I made to example__activity_stream (removing feature_json from array, snake-casing json keys) reasonable?

[tnightengale]

@tnightengale looking for the following feedback:

• Was not being able to specify a feature_json key as an included column a bug or did this functionality intentionally not exist? It seems like you intended to implement it since you had the json_unpack_key helper function already in place (asking for semantic versioning purposes)
• Does the abstraction I put in place make sense, or are there changes you'd like to see?
• Does this assumption (baked into the code make sense: that if a specified included column isn't identified as one of the standard columns (or project-specific aliases) from the Activity Schema spec, it is assumed to be contained in the feature_json?
• Are the semantic changes I made to example__activity_stream (removing feature_json from array, snake-casing json keys) reasonable?

1. It was not an intended feature, since folks could just add another CTE below and unpack the columns there. But this is a fine convenience feature.
2. The abstraction being the parse_column function? Yes I think it makes sense.
3. The assumption about an included column not in the default columns, being in feature json makes sense.
4. Lgtm!

[bcodell]

Thanks @tnightengale! I'm going to merge this then open a PR to bump the version to 0.4.0. This feature should provide a workaround to #26 and #33 until those bugs are explicitly resolved.