Do not configure to set `build.command`, `build.functions` and `build.publish`

[ehmicky]

In order to use this plugin, users have to set build.command, build.functions and build.publish in their netlify.toml.

We should instead re-use the configuration properties set the site:

• If build.functions is defined, we should use this. If not, we should default to the default functions directory. This value can be retrieved using constants.FUNCTIONS_SRC (see Functions UI build settings #11)
• Same for build.publish. This value can be retrieved using constants.PUBLISH_DIR.
• The main reason we require setting build.command seem to be to ensure that next-on-netlify is run after next build. This should already be covered by this plugin considering it calls next-on-netlify programmatically during onPostBuild.

This would require updating next-on-netlify so it accepts custom functions and publish directories.

The main pros would be:

• This allows users to define custom functions and publish directories
• This removes those configuration steps, so we can eventually provide a 0-steps use of this plugin

[ehmicky]

Done at #39.