Ninja status format string from argv.

[gkistanova]

Ninja supports a status format string from NINJA_STATUS env. This is a great feature to have.
Though, in an automated build environment provisioning environmental variables could be tricky.

This patch adds a support for a command line option (--status-format=) to set the status format string. If given, the status format string from the command line would override the one from NINJA_STATUS env.

Only a long option is supported for now. Easy to add a short form if it will be a use case for that.

[nicolasdespres]

If you want to set NINJA_STATUS just for a single process invocation, you can do env NINJA_STATUS='foo' ninja. Note that depending on your shell, you may even omit the call to the env program.

[gkistanova]

Right. Though, it is not always launched from a shell. Addressing that use case was the original motivation of the patch.

[nico]

Thanks, that looks like a good feature.

(Note to self: Related to #746)

[jhasse]

Though, it is not always launched from a shell.

env doesn't require a shell though :)

[gkistanova]

@jonesmz The PR has been rebased on the latest master branch.