reimplement `start` command

[Adman]

The programmer often wants to see some input values from sensor during testing, and it's so annoying to write the same command multiple times. So running our old friend start <cmd> <interval> would do the job

[mrshu]

@vargac is there any update on this?

[vargac]

@mrshu There was plan to improve vic this way, but no time. Anyhow, the plan still exists and perhaps pull request will come soon.