chmod bits linter

[mcandre]

• standard nginx static directories should not feature "other" writable bits
• ability to accept custom root directory paths
• warn for paths on non-UNIX file systems
• use stank to warn on unidiomatic executable permissions for POSIX-y shell scripts
• warn on unidiomatic permissions for SSH trees

[mcandre]

https://github.com/mcandre/carrots