feat: Bind exported methods

So that code like this, which is increasingly common, doesn't fail
because `this` isn't bound to the right object.

```
const {getType} = require("mime");

getType("hello.txt");
```

Mime.js

@@ -11,6 +11,10 @@ function Mime() {
   for (var i = 0; i < arguments.length; i++) {
     this.define(arguments[i]);
   }
+
+  this.define = this.define.bind(this);
+  this.getType = this.getType.bind(this);
+  this.getExtension = this.getExtension.bind(this);
 }
 
 /**