Add unicode printing support for barred output

[balki]

Add flag --barred-unicode that makes the output table prettier using unicode characters.

@johnkerl

1. Please confirm if the cli flag and implementation looks good. I will update docs and add tests.
2. Should there be reader support as well. If so, can that be deferred for later?

❯ echo "foo,bar\n1,2" | go run cmd/mlr/main.go --c2p --barred cat
+-----+-----+
| foo | bar |
+-----+-----+
| 1   | 2   |
+-----+-----+

miller on  barred-unicode via 🐹 v1.26.0-X:nodwarf5
❯ echo "foo,bar\n1,2" | go run cmd/mlr/main.go --c2p --barred-unicode cat
┌─────┬─────┐
│ foo │ bar │
├─────┼─────┤
│ 1   │ 2   │
└─────┴─────┘

miller on  barred-unicode via 🐹 v1.26.0-X:nodwarf5
❯ echo "foo,bar\n1,2" | go run cmd/mlr/main.go --c2p --barred cat | cat
+-----+-----+
| foo | bar |
+-----+-----+
| 1   | 2   |
+-----+-----+

miller on  barred-unicode via 🐹 v1.26.0-X:nodwarf5
❯ echo "foo,bar\n1,2" | go run cmd/mlr/main.go --c2p --barred-unicode cat | cat
┌─────┬─────┐
│ foo │ bar │
├─────┼─────┤
│ 1   │ 2   │
└─────┴─────┘

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