clean up REPL output (Fixes #451)

Example session:

```
[info] Running slamdata.engine.repl.Repl 
💪$ select distinct city from zips order by city
Mongo
db.zips.aggregate([
    { "$project" : { "__tmp1" : { "city" : "$city"} , "__tmp2" : { "0" : "$city"}}},
    { "$sort" : { "__tmp2.0" : 1}},
    { "$group" : { "value" : { "$first" : "$__tmp1"} , "__sd_key_0" : { "$first" : "$__tmp2.0"} , "_id" : "$__tmp1"}},
    { "$sort" : { "__sd_key_0" : 1}},
    { "$project" : { "city" : "$value.city" , "_id" : false}},
    { "$out" : "tmp.gen_0"}
  ],
  { allowDiskUse: true })
db.tmp.gen_0.find()

Results
{ "city" : "AARON"}
{ "city" : "AARONSBURG"}
{ "city" : "ABAC"}
{ "city" : "ABBEVILLE"}
{ "city" : "ABBOT VILLAGE"}
{ "city" : "ABBOTSFORD"}
{ "city" : "ABBOTT"}
{ "city" : "ABBOTT PARK"}
{ "city" : "ABBOTTSTOWN"}
{ "city" : "ABBYVILLE"}
...
💪$ set debug = 0
Set debug level: Silent
💪$ select distinct city from zips order by city
{ "city" : "AARON"}
{ "city" : "AARONSBURG"}
{ "city" : "ABAC"}
{ "city" : "ABBEVILLE"}
{ "city" : "ABBOT VILLAGE"}
{ "city" : "ABBOTSFORD"}
{ "city" : "ABBOTT"}
{ "city" : "ABBOTT PARK"}
{ "city" : "ABBOTTSTOWN"}
{ "city" : "ABBYVILLE"}
...
💪$ 
```
This is OK for now but long term there is a coupling here ('last element on right is the one we want') that we need to break in some principled fashion.