Add benchmark around string replacement.

Calculating -------------------------------------
                 #tr     62416 i/100ms
               #gsub     33750 i/100ms
              #gsub!     29695 i/100ms
                #sub     60774 i/100ms
               #sub!     64955 i/100ms
-------------------------------------------------
                 #tr   989348.8 (±5.0%) i/s -    4993280 in   5.060836s
               #gsub   422892.9 (±4.3%) i/s -    2126250 in   5.037741s
              #gsub!   364115.6 (±4.0%) i/s -    1841090 in   5.064496s
                #sub   964336.6 (±4.4%) i/s -    4861920 in   5.051775s
               #sub!  1016598.5 (±4.7%) i/s -    5131445 in   5.058987s

Author: parkr

benchmark/string-replacement

@@ -0,0 +1,13 @@
+require 'benchmark/ips'
+
+def str
+  'http://baruco.org/2014/some-talk-with-some-amount-of-value'
+end
+
+Benchmark.ips do |x|
+  x.report('#tr')    { str.tr('some', 'a')    }
+  x.report('#gsub')  { str.gsub('some', 'a')  }
+  x.report('#gsub!') { str.gsub!('some', 'a') }
+  x.report('#sub')   { str.sub('some', 'a')  }
+  x.report('#sub!')  { str.sub!('some', 'a') }
+end