Use buffer pool in StripHTML

Use buffer pool in StripHTML

Allocates less memory:

benchmark              old ns/op     new ns/op     delta
BenchmarkStripHTML     6572          6695          +1.87%

benchmark              old allocs     new allocs     delta
BenchmarkStripHTML     5              4              -20.00%

benchmark              old bytes     new bytes     delta
BenchmarkStripHTML     848           737           -13.09%

Compared to 0.12:

benchmark              old ns/op     new ns/op     delta
BenchmarkStripHTML     10210         6695          -34.43%

benchmark              old allocs     new allocs     delta
BenchmarkStripHTML     6              4              -33.33%

benchmark              old bytes     new bytes     delta
BenchmarkStripHTML     1456          737           -49.38%