Update requireMultipleVarDecl rule to support one var per declaration

`requireMultipleVarDecl` only supported one `var` per function scope and
consecutive `var` checks. It was not possible to check that each declaration
statement had it's own `var` terminated by a semicolon such as in the Felix's Node.js Style Guide. [Example.](Seravo/js-winning-style/blob/master/README.md#variable-declarations)

This means the original `requireMultipleVarDecl` option of boolean true for
consectutive `var` check is now changed to a string type aptly named
`"nonconsecutive"`, and a new option of `"onedecl"` is introduced for the new
check.

With
```js
"requireMultipleVarDecl": "onedecl"
```
the variable declaration style of
```js
var x = 1;
var y = 2; var z = 3;
```
will pass the style checks while that of
```js
var x = 1,
    y = 2;
```
will fail.