Add csvkit command to display help message about other commands

[hiAndrewQuinn]

Heya! Big fan of csvkit here, it's been one of the packages I install with Shell Bling Ubuntu for a long time now. 😃

I think it would make sense to wrap the command csvkit itself around the other real commands it installs, so that running csvkit by itself prints something like

$ csvkit
csvkit 1.0.7 installs the following commands:
- csvclean
- csvcut
- csvformat
- csvjoin
- csvjson
- [... you get the idea]

This is a very small thing, but I bet a lot of people trying it out for the first time would appreciate it.

[jpmckinney]

I would accept a PR.

[hiAndrewQuinn]

adding to my to-do list 👌 glad to know you're game!