Add a few benchmarks

[koppen]

Run them with rake benchmark:

#add with many distinct tokens             n=500     0.0012s  n=4000    0.0667s  ratio=  54.72  max= 128.00  OK
incremental + and render on each step      n=300     0.0017s  n=2400    0.0852s  ratio=  50.26  max= 128.00  OK
chained + with a single render at the end  n=300     0.0007s  n=2400    0.0280s  ratio=  38.69  max= 128.00  OK
#remove with many distinct tokens          n=500     0.0018s  n=4000    0.1076s  ratio=  59.70  max= 128.00  OK
#toggle with many distinct tokens          n=500     0.0024s  n=4000    0.1329s  ratio=  55.74  max= 128.00  OK