Check if the default value supplied is a slice and not nil, if so
print the list of values supplied.
Test case for slice argument with and without default values.
Default values for slices was not printed because slice is not
comparable, but the zero value for slices is nil.

We try to compare []strings, which are uncomparable types:

`panic: runtime error: comparing uncomparable type []string`

Check if the value isn't it's zero value and if not add a default value
to the usage text.

Adds help to the options in help output with an easy way to add more
built ins.

Only the usage message was written to stderr, the error was written with
the standard fmt.Println.

When the parsing of parameters/flags fails eg. when a required flag is
missing, print the usage statement and error to stderr instead of
stdout.