Support flags defined in a container object

[t-kalinowski]

Proposed syntax:

#| flags-dest: ::FLAGS

FLAGS <- local({
  noise <- .2
  batch_size <- 32

  environment()
})

...

model %>% fit(batch_size = FLAGS$batch_size)

Alternative ways to supply the flags-dest param might include:

#| flags-dest: ::FLAGS
#| flags-dest: .::FLAGS
#| flags-dest: ./train.R::FLAGS

Also, a decision regarding the meaning of 1 vs 2 colons needs to be made. E.g., scripts that implicitly define pipelines may use a single : to refer to a named operation, while two colons :: may be used to refer to R objects. Alternatively, we can use $, or ., if :: is to be reserved for guild created packages.