Complexity refactoring

[mestachs]

First thanks for the gem !
I worked on the style, complexity and duplication in the current version.
I've added

• sqllite support (not sure if the data are accurate but do the job in development).
• made the created_at column configurable (can become eg last_viewed_at)

Before

flog

114.8: CohortMe::analyze lib/cohort_me.rb:5

flay

1. Similar code found in :lasgn (mass = 34)
   lib/cohort_me.rb:45
   lib/cohort_me.rb:47

After

flog

56.1: CohortMe::analyze ./lib/cohort_me.rb:6
30.0: CohortMe::as_table ./lib/cohort_me.rb:42

flay

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