Feature request: support for attributes

I know it can be tough, since Julia doesn't support anything similar, but many people (including me) store important metadata in attributes.

I guess objects' metadata should be inserted as distinct items in the resulting dictionary.

E.g. if I read object a generated as

a<-"text"
attr(a, 'label') <- 'my label'
save("a", file="obj_a.rda")

then it should produce dictionary with two entries:

1. "a" → "Kuku!"
2. "a|label" → "my label"

Character | can be replaced with any other character that cannot occur in variable's name.

No file or test is named; start by locating the RData reader entry point and inspecting how object values are converted into the resulting dictionary. Confirm how R attributes are represented, then add coverage for the example's value and label entries, with completion defined as preserving attributes as distinct dictionary entries.