[RFE] Support periodically printing keep-alive message on no output

[mgorny]

When running very slow & quiet commands, user can see no output for quite a long time. Besides this being confusing, it e.g. causes the NetBSD LLVM buildbots to assume timeout due to no output from the running jobs. This is especially problematic given that ninja -v doesn't forward the output from the command until it finishes.

I think it would be helpful if ninja had an option to enable periodic status reports. For example, if no commands finished in the last N minutes, it could print something like ' still running: <num/count> <current-job(s)>'.

[jhasse]

Duplicate of #111.