Return counter value after incrementing/decrementing

[pbstriker38]

• Redis multi return an array with the responses of each command that
  was run. Use [-1] to get the last value of the array and return it.

closes: #62

[pbstriker38]

I wasn't sure if we wanted to go for readability or performance. I can switch this to use .last if it's more clear.

response = [false, 4]
Benchmark.ips do |x|
  x.report("[1]")  { response[1] }
  x.report("[-1]")  { response[-1] }
  x.report("last") { response.last }
  x.compare!
end
Warming up --------------------------------------
                 [1]     2.138M i/100ms          
                [-1]     2.058M i/100ms          
                last     1.857M i/100ms          
Calculating -------------------------------------
                 [1]     21.546M (± 2.6%) i/s -    109.040M in   5.064581s
                [-1]     21.314M (± 2.2%) i/s -    107.028M in   5.024219s
                last     18.051M (± 5.1%) i/s -     90.974M in   5.054999s
                                                 
Comparison:                                      
                 [1]: 21546133.6 i/s
                [-1]: 21313633.8 i/s - same-ish: difference falls within error
                last: 18050678.0 i/s - 1.19x  (± 0.00) slower

[jeremy]

#last does read better, but this works. Thanks @pbstriker38!