add example field to scribeCommandArgs

[jmbarbone]

Should be able ot include examples and append to description

library(scribe)
ca <- command_args()
ca$add_argument("foo")
ca$add_description("stuff here")
# add examples one-by-one; parameters for appending `"$ "` or `"> "` or `""`
# multiple examples should have an option for aligning?
ca$add_example("commamd value", "single value")
ca$add_example("command value value", "two values")
ca$help()
#> {scribe} command_args
#> 
#> file : {path}
#> 
#> DESCRIPTION
#>   stuff here
#> 
#> USAGE
#>   {command} [--help | --version]
#>   {command} [--help] [--version] [foo [ARG]] 
#> 
#> ARGUMENTS
#>   --help    : prints this and quietly exits                   
#>   --version : prints the version of {scribe} and quietly exits
#>   foo [ARG] : help information for foo  
#> 
#> EXAMPLES
#>   $ command value       # single value
#>   $ command value value # two values