@kha It will be awesome to automate the following idiom with a macro :)
```
def defIndent := 4
def getIndent (o : Options) : Nat := o.get `format.indent defIndent
@[init] def indentOption : IO Unit :=
registerOption `format.indent { defValue := defIndent, group := "format", descr := "indentation" }
```
We can use this primitive to process command line arguments of the form
`-D <key> = <value>`
TODO: allow users to attach `[init]` to definitions of the form
```
@[init] def foo : IO Unit := ...
```
and avoid the awkward auxiliary constant.
