Block API: Block splitting, insert after

[aduth]

Related: #374
Slack discussion: https://wordpress.slack.com/archives/C02QB2JS7/p1491308809656882 (cc @spocke)

With current block APIs, we might be able to support this by passing additional callbacks to the edit render handler, e.g. insertBlockAfter.

Proposal:

insertBlockAfter( block: ?WPBlock )

• If passed null, inserts empty block after currently selected block
• If passed block, appends block after currently selected block
  • How does a block implementation know how to create a block node? Should we overload this to support passing e.g. markup string?

In all cases, moves cursor focus to start of newly inserted block.

[mtias]

In the context of the inserter, I was thinking we may want to have an action for insert block that is capable of handling any insertion position within the list of blocks in state.

[youknowriad]

Closing since this is already done