Add simple benchmarks
Add simple benchmarks

At a first glance, it can be seen that Relaxed is about 5x slower than Strict:

BenchmarkStrictEmpty     1000000              2156 ns/op
BenchmarkStrictSingle     200000             10592 ns/op
BenchmarkStrictMany        50000             31996 ns/op
BenchmarkRelaxedEmpty     200000              9573 ns/op
BenchmarkRelaxedSingle     20000             71685 ns/op
BenchmarkRelaxedMany       10000            166668 ns/op

digideskio/xurls,mvdan/xurls