Rewrite explicit-length-check rule

[fisker]

Updated logic,

the old logic:

report problems one case by one case

the new logic,

1. If node matches wanted style, done.
2. If node is checking non-zero/zero length, directly fix to wanted style
   in this way we don't need add fix for any new case, only need add check logic.

---

With the new logic, the following changes are easy to be done:

• Added auto-fix for all cases
• Check == and != operator too
• Forbid logic like if (!(foo.length === 0)) {}, we can know it's checking non-zero, it should be if (foo.length > 0) {}
• Added yoda style check

Other changes,

• Simplify messages, just tell what should use.

For case if (foo.length) {}

- `length` property should be compared to a value.
+ Use `.length === 0` when checking length is zero.

For case if (!foo.length) {}

- `length` property should be compared to a value.
+ Use `.length > 0` when checking length is not zero.

Fixes #794

[sindresorhus]

Very nice!