Download and search FreeBSD build cluster fallout logs.

`fallout fetch` downloads build failure logs so they can be accessed and
searched from the local file system. It organizes these logs into a directory
hierarchy under ~/.cache by the builder name and port origin.

`fallout grep` finds failure logs matching a given set of conditions and
regular expressions. Combining it with portgrep allows advanced
searching for e.g. specific maintainer, failures of ports with specific USES,
with specific dependency, etc.