SemVer Patch

• add flush api to default stdout/stderr logger (#87)
• Fix typo in LogHandler doc comment (#76)
• Link to ianpartridge/swift-log-syslog backend impl (#73)
• Fix documented ordering of Level (#78)
• Add Adorkable/swift-log-format-and-pipe to Backend list (#83)
• test formatting issues as part of sanity check (#84)
• better documentation around MetadataValue being expressible by literal
• Add HeliumLogger to README (#88)