Allow just-in-time loading

PR for #42

#### To use jit-grunt instead of load-grunt-tasks plugin, just set ```jitGrunt: true```

```js
require('load-grunt-config')(grunt, {
  jitGrunt: true
});
```
or specify ```jitGrunt: {}``` configuration object 
```js
require('load-grunt-config')(grunt, {
  jitGrunt: {
    // jit-grunt configuration here (can be just empty object)
  }
});
```

This change should not affect current users.

#### Note:
jit-grunt is using task name to find and load grunt plugins, so in some cases you will need to specify [static mappings](https://github.com/shootaroo/jit-grunt#static-mappings).

**For example:**
```js
jitGrunt: {
    simplemocha: 'grunt-simple-mocha'
}
```