(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
```
