Parse command line arguments based on tag annotations on struct fields (library)

 This is a library to parse command line arguments based on tag
 annotations on struct fields. It came as a spin-off from aerc to deal
 with its internal commands.
 .
 This project is a scaled down version of https://github.com/alexflint/go-
 arg with different usage patterns in mind: command parsing and argument
 completion for internal application commands.