add -max-imports and -max-exec-time option

[fujiwara]

refs #17

$ rin  -help
Usage of rin
  -max-exec-time duration
        max execution time duration in batch execution (e.g 60s)
  -max-imports int
        max number of imports in batch execution

In batch mode, Rin breaks import action when max-exec-time or max-imports reached.