Refactor into OOP and implement Thor

__Caveats:__
I don't think the CURRENT requirements would _necessitate_ the new design, but I think these changes should make it easier to expand the requirements in the future when the need for change arises. I did this partially to practice OOP design ideas I've been learning.

__I am certainly open to improvements on this design!__

## Overview
This change makes use of a more robust command line solution (thor) and makes the solution more maintainable by introducing an oop design. Also will require a rewrite on some of the documentation (which this commit does not include.) I was going to use a Rakefile initially, but that would involve some ugly syntax in order to allow arguments to be passed. Note: Thor may be overkill for this project. OptionParser is always there.

## In Brief
Run `Thor list` to see available commands. If you don't have the [thor gem](https://rubygems.org/gems/thor), install it.

## Details
* The Thorfile requires Thor to run. (`gem install thor`). It provides two tasks, `Thor page:new TITLE CATEGORY` and `Thor page:relink`. To see all the tasks and their descriptions, run `Thor list` in the terminal.
* A few notes on the `Thor page:new` task- __it now requires both title and category__. Before they were optional.
Also, Thor page:relink deletes all the files in _pages and regenerates them.
* The lib/helpers.rb file provides a Helpers module and a Page class. This
module could be expanded to include other helper classes.