Allow scientific notation in DSL literals; mlr bar --auto

• Miller has always supported scientific notation in field values, e.g x=1e6. However, it had never supported scientific notation in DSL literals, e.g. mlr put '$y = $x + 1e6. This release fixes that.
• Additionally, mlr bar now has a ---auto flag which holds all records in memory and computes limits from the data, so you don't have to compute them separately and pass them in via --lo and --hi.