Improve `pxl init` command

[duijf]

The current setup is done through the pxl init command, but there are a few things we can improve about its UX:

• The init command SHOULD verify the configuration settings before saving them to a file.
• The init command MAY offer to create the given bucket if it doesn't exist yet (after user confirmation)

[duijf]

Postponing this until after we have something for https://github.com/svsticky/pxl/issues/15, https://github.com/svsticky/pxl/issues/34 and https://github.com/svsticky/pxl/issues/35 as those issues touch the config which init is creating.