Add an option of timeout duration for executing command

[taku-k]

What

This pull-request introduces the option for specifying a duration after which a command execution will be timeout.

Why

Running a slightly heavy check command will be easily timeout, so users should be able to adjust the timeout duration.

[Songmu]

That's a good pull request! We wanted to implement it for a long time.

I point out two points.

Option name

I think timeout_seconds would be better.

naming context is confusing

I think it is better not to use context in naming except for the standard context package
in Go world.

How about naming it as CommandOption and adding it to the end of the argument?

(In the first place, the package name util is not good itself. There is room for
improvement there.)

[taku-k]

Thank you for the review. I followed your advice.

[Songmu]

Thanks for quick fixing.

One more. You should set timeoutDururation for action in buildCheckPlugin.

[taku-k]

I miss it. Thank you.

[Songmu]

Thanks! I