For accessibility reasons, we should limit our lines to 100 chars max.
https://github.com/slack-ruby/slack-ruby-client/pull/293#discussion_r309472083

Disable Metrics/BlockLength in specs because the length of a block in
the test suite isn't something we want to lint. We want the tests to be
as long as they need to be.

Set an explicit line length metric instead of continually updating this
as we go. Let's pick a max line length that we want to see and stick
with it. This metric should only ever decrease: we don't want to see it
ever increase again.

This is a big step forward in our Rubocop setup. I addressed all of the todos
from the current version of Rubocop that made sense to. The only things that
remain are metrics and one cop that relies on the line length metric to work.

I made some judgment calls on disabling a few cops:

1. The `Layout/IndentHeredoc` cop wants you to either use the squiggly heredoc
   from Ruby 2.3 or introduce a library. Since we are a low-level library that
   is used as a transitive dependency, we cannot introduce another library as a
   dependence, so that option is out. Also, we support Rubies back to 2.0
   currently, so using the squiggly heredoc isn't an option. Once we remove
   support for Rubies older than 2.3, we can switch to the squiggly heredoc cop.
2. The `Naming/FileName` cop was reporting false positives for a few files in
   the repository, so I disabled it on those files.
3. The `Style/DoubleNegation` cop reports lints on a few cases where we use
   double negation. Given the very generic nature of Hashie, the double-negation
   is the easiest, clearest way to express that we want an item to be a Boolean.
   I disabled the cop because we exist in the gray area where this makes sense.

- Updated, and enhanced specs
- Improved documentation

In Python a "Hash" is called a "Dictionary", and ...

> "It is an error to extract a value using a non-existent key."

See: https://docs.python.org/2/tutorial/datastructures.html#dictionaries

EXAMPLE:

    class StrictHash < Hash
      include Hashie::Extensions::StrictKeyAccess
    end
    >> hash = StrictHash[foo: "bar"]
    => {:foo=>"bar"}
    >> hash[:foo]
    => "bar"
    >> hash[:cow]
      KeyError: key not found: :cow