v1.18.0

Added

• ./runtests test runner with coverage tracking by default
• testing on OS X
• testing on Windows
• UintFlag, Uint64Flag, and Int64Flag types and supporting code

Changed

• Use spaces for alignment in help/usage output instead of tabs, making the
  output alignment consistent regardless of tab width

Fixed

• Printing of command aliases in help text
• Printing of visible flags for both struct and struct pointer flags
• Display the help subcommand when using CommandCategories
• No longer swallows panics that occur within the Actions themselves when
  detecting the signature of the Action field