support for disabling/enabling of tests based on #[cfg(target_os = ...)]

[budziq]

Currently rust-cookbook syslog example is an awful hackjob due to skeptic not bing able to selectively run tests that make sense only on selected #[cfg(target_os] (syslog makes sense only on linux wile the example is sprinkled with ifdefs to make it compile everywhere).

Skeptic should be able to distinguish which examples make no sens and skip building these (most likely with snippet annotation similar to the no_run or ignore such as cfg_target_os_linux)