Plan for 3.0.0

[LinusU]

Things I want to get in:

• Throw on invalid schema format #162 - Throw on invalid schema format
• fix: handle custom formats with null values #161 - Handle custom formats with null values
• Minor tweaks regarding required: true (encourage v4-style required) #53 - Minor tweaks regarding required: true
• Drop support for Node.js < 6

I would also be open to dropping support for any schema verison below 4.

Anything else that anyone would like to see fixed/improved/added? Please only bring up stuff that would actually be a breaking change. Fixing bugs should be done in patch releases.

[jimmywarting]

Since node 6 will be used:

• replace xtend with Object.assign