Context based commands

[shikaan]

Sometimes you need to execute different commands in different environments, but you don't necessarily want to expose the information through commands

We could annotate those commands with the context

@Linux
build:
  nasm -g -f elf64 -o build/$1.o $1

@Darwin
build:
  nasm -g -f macho -o build/$1.o $1

Not sure about the syntax, since I have no idea if this should be more generic