differing flexi column configs between addons can cause conflicts

[kybishop]

We can likely fix this through https://github.com/html-next/flexi-dsl/issues/3, which would allow addon authors to add a column suffix if they don't use the default 12-column configuration.

[kybishop]

I'm dumb, we already have a column prefix option in the config, which can be used to avoid this issue. That said, we might want to issue a warning (in dev mode) if the following is true:

1. we're in an addon using flexi
2. the addon uses a non-standard number of columns
3. the addon does not specify a column prefix

@rwwagner90 thoughts?