Metric Update refactor

[alisonmyers]

• now includes util tests using jest
• added standard linter
• uses natural library for word count
• rounds to 2 decimals (and ensures all summary output is rounded to 2 decimals for consistency)
• removes summary by module - discussed as too specific, making code harder to maintain
• transforms all "datetimes" to luxon DateTime when data is loaded and uses luxon for date calculations
• transforms all output datetimes to Pacific Time when writing to csv
• simplifies "time" metric to use days - see README
• resolves (minor style comment) I think the wordCounts and timeDiffs functions could be more consistent. In wordCounts, we have a getWordCount function, and then wordCounts is a one-liner to apply .map(). The logic is generally the same for timeDiffs, but an anonymous function is used inside the map. #33
• adjusts naming conventions and explanations: posts & replies are referred to as responses (for most use cases this may seem trivial to the end user, but I think there is usefulness in future to distinguish between a discussion_topic, a post and a reply see README.