v0.2.0: Add `--duplicate-columns`

The `--replace` flag from v0.2.0-alpha.1 has been removed. Instead, you
may now specify one of the following:

`--duplicate-columns=error` (default):
    Fail if any output columns already exist in the input CSV.
`--duplicate-columns=replace` (default):
    Remove any input columns which have the same name as output columns
    before geocoding.
`--duplicate-columns=append` (default):
    Just add the output columns and don't worry about duplicates.

Users migrating from 0.1.x should specify `--duplicate-columns=append`
if they want to maintain backwards compatibility.

This version also ships with a more extensive set of integration tests,
which actually run `geocode-csv` as a CLI tool, and which use the real
SmartyStreets API.