Add benchmark test for StripHTML

Add benchmark test for StripHTML

go test -test.run=NONE -bench=".*" -test.benchmem=true ./helpers

Old vs new impl (string.Replace vs string.Replacer):

benchmark              old ns/op     new ns/op     delta
BenchmarkStripHTML     10210         6572          -35.63%

benchmark              old allocs     new allocs     delta
BenchmarkStripHTML     6              5              -16.67%

benchmark              old bytes     new bytes     delta
BenchmarkStripHTML     1456          848           -41.76%