New switch to specify different local config file
Add documentation for `box` command

A few manpages describing `box` command itself as well as its
configuration file `box.conf`. The man pages are written in mdoc
form.
Change print error function name
Use internal 'printf'
Refine the error message format
Return instead of exit from function
Switch to using getopts
Reflect new -a flag in docs and usage
Show usage on error
Bail when no action specified