sysexits 1.0.6

A port from <sysexits.h> promoting standardization of exit codes.

1.0.6

• Added usage example to README.md and links to dart references on using exit / return codes

1.0.5

• Switch to pana for ci

1.0.4

• Removed dart 1.x from ci

1.0.3

• Updated to min dart sdk 2.12 for null saftey
• Updated comments to dartdoc style

1.0.2

• Updated README.md formatting

1.0.1

• Added ci support for dart SDK 1.x

1.0.0

• Port from <sysexits.h>