## Utils:
Data.frame-like object:
- [x] ess-r-insert-obj-dt-name

Column/Variable name: with `C-u C-u`, it prompt for the dt name for search in.
- [x] ess-r-insert-obj-col-name
- [x] ess-r-insert-obj-col-name-all

Column/Variable name: with `C-u C-u`, it prompt for the dt name for search in, or
with `C-u`, it prompt for column/variable name to search in.
- [x] ess-r-insert-obj-value
- [x] ess-r-insert-obj-value-all

## Customization
### ess-r-insert-obj-complete-backend-list
- jsonlite
### ess-r-insert-obj-read-string
- ess-completing-read (default)
- completing-read
- ido-completing-read
- ivy-completing-read