New i argument

Many thanks to @janwillemtulp for the addition of the i argument in lifecycle callbacks:

• #86
• #87
• #88

v3.0.0

• v3 API Changes #70
• (bug fix) Datum passed to destroy is now the most recent #81
• Source code is now commented and optimized for readability.

v2.5.1

• Linting all-in #78 - Adopting ESLint, and some minor code rearrangement.

v2.5.0

• es2015, rollup config file, and source maps #73 (thanks @micahstubbs !)
• Add links to es2015 examples.

v2.4.0

Optimize children selection #66

v2.3.0

Accept DOM Node #60

v2.2.0

More efficient context handling #59

v2.1.0

• Return merged selection #56

v2.0.0

• Use names create, render, destroy #48
• Bring back className argument #47
• Pass additional "context" object #49
• Updated examples to use Redux

v1.0.0

• API Overhaul - More D3-like API #40
• Use enter, update, exit names #42
• Removal of local state feature