Book: Chapter on "writing robust CLI applications"

Inspired by @vorner's comment (https://github.com/rust-lang-nursery/cli-wg/pull/105#discussion_r240003491).

We should look into adding an in-depth chapter on how to write applications in a robust and well-behaved way. For example, when an application was forcefully terminated (kill -9, power outage, …), it should still be possible to start it again without any issues.

For this to work, several aspects need to be considered. For example,

• cleaning up or overwriting temporary files from a previous run without issues
• atomically writing config files so they are not corrupted

Other "robustness" aspects that come to mind:

• Follow sane resource limits for the current OS (e.g. not open millions of file descriptors on macOS)
• Try to be both forward and backward compatible with config files and CLI arguments

Noe: As @vorner correctly pointed out, this does not need to be specific to CLI applications!

No target file or test is named. Review the book's existing chapter structure and the linked pull request discussion first, then define the chapter scope around the listed robustness topics. Done means the agreed in-depth guidance is added to the book and fits its existing organization.