Specifying compiler executable in config.nims

[exelotl]

Hey! The nimscript way of doing this is [put(key, 
value)](https://nim-lang.org/docs/nimscript.html#put%2Cstring%2Cstring)

For example in my project I'm doing:
    
    
    put "arm.standalone.gcc.path", getEnv("DEVKITARM") & "/bin"
    put "arm.standalone.gcc.exe", "arm-none-eabi-gcc"
    
    
    Run