Webpack Loader API Support

Compatible loader features:

• this.emitWarning
• this.emitError
• this.addDependency
• this.addContextDependency
• this.clearDependencies
• this.callback style for yielding code + map
• async this.callback style for yielding code + map
• synchronous return style for yielding code
• pitch loaders
• this.cacheable (no op, we're always caching)
• this.exec (??)
• this.resolve
• this.resolveSync

Loader context:

• this.context
• this.query
• this.resourcePath
• this.resourceQuery
• this.options (somewhat working, we don't serialize the plugins though or anything with cyclic dependencies)
• this.data (pitch)
• this.loaders
• this.loaderIndex
• this.values
• this.inputValues
• this.debug
• this.minimize
• this.sourceMap
• this.target
• this.webpack
• this._compilation
• this._compiler

Striked out stuff won't be done.