Rename --limit to --config, add --help-config

[simonw]

#270 introduced --limit but on further thought it should be called --config instead.

--page_size should becomes --config default_page_size:1000

Add --help-config to show full help showing all config settings.

[simonw]

Could also support these as optional environment variables - DATASETTE_NAMEOFSETTING

[simonw]

http://datasette.readthedocs.io/en/latest/config.html