Permit to use project directory name in build_dir reho.toml

[yodatak]

Thanks to make this awsome app i use a lot to different texts and i want to use a single rheo.toml but i can because

i want the output dir to be

build_dir = "../../exports/iwwtext1/"

build_dir = "../../exports/iwwtext2/"

and for now it seem set to

build_dir = "../../exports/[project directory name]/"
don't seem to work

thanks !