keepWarnings option like in postcss-log-warnings

[gazay]

I think by default we better not clear warnings after append them to result because there can be used other plugin after, like postcss-log-warnings. So option should be named as clearWarnings

[MoOx]

+1. I plan to integrate messages api in cssnext soon and I will really need to get both methods at the same time :)

[ai]

I agree that clearWarnings option is better, because it is OK if we will show warning i browsers and CLI too.

[MoOx]

+1 for clearWarnings. Maybe @davidclark could do the same for postcss-log-warnings.

[davidtheclark]

I'm fine with this. Created in issue in log-warnings to get it done.