Add parseBoolish utility function

Will be used to support stuff like  `--flag=yes`.

(Not sure if it will stay shared.)
Add test for parseBoolish utility function
Allow boolean values for flags

Not supported by parseArguments() yet.
parseArguments: Support --flag=no
Add test for --flag=no
parseArguments: Support --no-flag
Add test for --no-flag
Merge branch 'booleanFlags'

Support for boolean flags, allowing stuff like

    --no-ff
    --ask=no