Drop defaultAttrs

[tgvashworth]

API breaking change.

The interaction between attributes and defaultAttrs is unclear and open to hard-to-spot bugs, for example when a mixin uses the a different form to the component.

Support for defaultAttrs should be dropped, breaking the API and forcing everyone to move the attributes.

[tgvashworth]

Thinking this could be done by making attributes and defaultAttrs compatible (currently they're mutually exclusive).

[giuseppeg]

Yeah we could have a flag on attributes say strictmode to disable the strict checks introduced by this method. In this way defaultAttrs would only be a proxy fn to attributes with strictmode: false

[tgvashworth]

Enough time has passed for defaultAttrs to have been migrated away from, so the deprecation strategy will be:

• release a 1.x that spits warnings (in debug mode) when defaultAttrs is used
• make defaultAttrs a warning-spitting (regardless of debug mode) no-op in 2

We need to be this strict about it because the two are mutually exclusive, so projects like TweetDeck will have to do an all-at-once switch.