Implement AMD-style plugins for `load` and `load/webpack`.

[mwistrand]

Updates load.ts and load/webpack.ts to allow amdjs-style plugins to be used.

Resolves a portion of #201.

// html-dom-plugin
import { Load, useDefault } from '@dojo/core/load';

export function normalize(resourceId: string, resolver: (mid: string) => string): string {
    return resolver(mid).replace('app-name', 'appName');
}

export function load(mid: string, load: Load): Promise<any> {
    return load(mid).then(useDefault).then((value: string) => htmlToDom(string));
}

load('html-dom-plugin!path/to/html/string').then((htmlTree) => {
    // ...
});

[codecov-io]

Current coverage is 95.19% (diff: 94.11%)

Merging #271 into master will increase coverage by 0.08%

@@             master       #271   diff @@
==========================================
  Files            34         34          
  Lines          1819       1851    +32   
  Methods          19         19          
  Messages          0          0          
  Branches        355        366    +11   
==========================================
+ Hits           1730       1762    +32   
- Misses           27         28     +1   
+ Partials         62         61     -1   

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