CONSISTENCY: Add CLI argument --expr=EXPR complementing -e EXPR

[HenrikBengtsson]

All short-form command-line options for R and Rscript has a long-form counterpart (e.g. -f FILE and --file=FILE), except -e EXPR. For completeness, it would be nice if --expr=EXPR was supported too. One argument for this is that Rscript does not support any of the short-form CLI options, except -e EXPR.