Better output formatting

[sj14]

Try to format the output better, e.g. with https://golang.org/pkg/text/tabwriter/

Currently it looks like this:

inserts:        7.010234901s    280409  ns/op
selects:        1.193978836s    47759   ns/op
updates:        7.061338251s    282453  ns/op
deletes:        6.68747513s     267499  ns/op
relation_insert0:       7.045241106s    281809  ns/op
relation_insert1:       9.43514862s     377405  ns/op
relation_select:        2.499483223s    99979   ns/op
relation_delete1:       8.389839477s    335593  ns/op
relation_delete0:       8.026760922s    321070  ns/op
total: 57.349712525s