Crude confirmation screen

The idea is to present the user with all the values he entered before
writing the sketch, to give a chance to modify things.

This is not finished, just a first crude attempt.
Removed old sketcify command definition
Reworded env/metadata parameter messages
Enforce one-line JSON in cfsketches.json

Under some circumstances the JSON output in cfsketches.json was produced
in pretty-printed form (multi-line and indented), which broke things. We
now explicitly set the format to one-line before writing it.
Split Sketchifier class

Split the Sketchifier class into its own file and improve the query_spec
data structure.
Improved confirmation screen

Parameters are now pretty-printed.
Fixed namespace insertion and sketch merging

Namespace insertion was not working properly. Also moved merging of data
into the new sketch structure to right before writing the files, so that
they would be correct if the user makes any changes in the confirmation
screen.
Merge pull request #434 from zzamboni/sketchify_36

Implemented confirmation screen and other fixes.