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"}
...
💪$ 
```
Yes, that makes sense. Maybe the PhaseResult carry an "importance" signifier (which is maybe just debug level by another name). That would useful to the client as well, when we build that UI.