fix: set rscript repo path to current working directory or CRON_LIVE env var

[hedsnz]

• the RscriptRepository argument of cron_rstudioaddin defaults to the CRON_LIVE environment variable, or the working directory if unset.
• add verify_rscript_path function that checks whether the supplied RscriptRepository path exists and that the user has permissions to write to it.
• update documentation describing these changes.
• addresses https://github.com/bnosac/cronR/issues/37
• more discussion here https://github.com/bnosac/cronR/pull/40

[jwijffels]

Thanks.

[jwijffels]

Updated package is now on CRAN. Good luck with your deployments.

[hedsnz]

Thanks :)