Allow variable substitution

[psolymos]

Currently we have the !expr method as used by yaml etc.

There are other options, like SPel (#{...}) and ${...} interpolation (runtime vs pre-run).

https://cran.r-project.org/web/packages/GetoptLong/vignettes/variable_interpolation.html also allows ${...} and @{...}.

• use ${...} for environment variables
  • allow the use of defaults etc: https://tldp.org/LDP/abs/html/parameter-substitution.html and https://docs.docker.com/compose/environment-variables/
• use @{...} (or #{...}) for runtime (but unlike like !expr): this would insert value into a character
• !expr will override the whole prop, so can result in numeric etc

For environment variables, allow .env and .Renviron read from the project dir or be listed as part of --file or --env-file

See also:

• https://stringr.tidyverse.org/reference/str_interp.html
• https://github.com/renkun-ken/rprintf