take advantage of click's type validation/coercion

(saves us having to write the stuff ourselves). Also adds a small
click plugin to do datetime parsing.

Sample output:

```
[leohemsted:~/dev/api]$ flask command create_provider_rates --help
Usage: flask command create_provider_rates [OPTIONS]

  Backfill rates for a given provider

Options:
  -p, --provider_name [mmg|firetext|ses]
                                  [required]
  -c, --cost FLOAT                Cost (pence) per message including decimals
                                  [required]
  -d, --valid_from DATE           [required]
  --help                          Show this message and exit.
[leohemsted:~/dev/api]$ flask command create_provider_rates -p ses -c 1.234 -d invalid
Usage: flask command create_provider_rates [OPTIONS]

Error: Invalid value for "-d" / "--valid_from": Could not parse datetime string "invalid" formatted as %Y-%m-%dT%H:%M:%S
```

requirements.txt

@@ -7,6 +7,7 @@ Flask-Marshmallow==0.8.0
 Flask-Migrate==2.1.1
 Flask-SQLAlchemy==2.3.2
 Flask==0.12.2
+click-datetime==0.2
 gunicorn==19.7.1
 iso8601==0.1.12
 jsonschema==2.6.0