Allow inverted cfg attributes, like `[not(macos)]` about just

[mkatychev]

reposting #604 (comment):

[@]casey are there any discussions on doing cargo style [not(condition)]/[condition] annotations?:

[not(windows)]
run:
  cc main.c
  ./a.out
  
[windows]
run:
  cl main.c
  main.exe

I imagine my usecase is quite common for some when dealing with doing/avoiding setup specific to CI environments:

[env_var("CI") == "true"]
run:
  ./do_the_ci_setup.sh
  ./run.sh

[not(env_var("CI") == "true")]
run:
  ./run.sh