Improve editable API

[peyerluk]

Proposed new API:

// Events:

beforeFocus: function(block) // only do as config option?
focus: function(block)
beforeBlur: function(block) // only do as config option?
blur: function(block)
selection: function(block, selection)
cursor: function(block, cursor)
empty: function(block)
move: function(block, selection, direction) // what is this again? Do we need it?
change: function(block)
stoppWriting: function(block)
insert: function(block, cursor)
split: function(firstBlock, secondBlock, cursor)
merge: function(block, cursor)
newline: function(block, cursor)
switch: function(block, cursor) // is this needed or covered with blur and focus?
paste: function(block, cursor, pastedContent)
cut: function(block)
copy: function(block, selection)

// Actions:

insertBlock: function(block, direction, cursor, callback)
splitBlock: function(block, cursor, callback)
mergeBlocks: function(block, direction, cursor, callback)
switchBlock: function(block, direction, cursor, callback)
filterPastedContent: function(block, pastedContent)
InsertNewline: function(block, cursor, callback) // only do as a config option?
// alternative naming: 
doInsert: function(block, direction, cursor, callback)