Allow setting offset in directive (#7)

* Allow setting offset in directive

The change to the implementation is:

```html
<a href="#" v-scroll-to="{ 'scrollTo': '#section-' + index, 'offset': -75 }" class="nav-link">{{ link.name }}</a>
```

Let me know what you think!

Closes #6

* Update index.js

Change scrollTo to el and make object optional, if you have any other changes feel free to update :) thanks!

* Update index.js

This makes more sense..

Author: onedr0p

index.js

@@ -28,9 +28,16 @@ exports.install = function (Vue) {
     function handleClick(e) {
         e.preventDefault()
 
-        exports.scrollTo(this.value, 500, {
+        if (typeof this.value === 'object') {
+          exports.scrollTo(this.value.el, 500, {
+            easing: exports.easing['ease'],
+            offset: this.value.offset
+          })
+        } else {
+          exports.scrollTo(this.value, 500, {
             easing: exports.easing['ease']
-        })
+          })
+        }
     }