Support tsconfig.json files

[jbrantly]

We should try to add support for tsconfig.json files prior to the release of TS 1.5. See microsoft/TypeScript#1692

Part of this change should also include updating the additionalFiles configuration to just be files which matches what is in the configuration file.

[mgutz]

+1

[mgutz]

FYI, here's the function I use in my project.

/**
 * Convert tsconfig.json file to ts-loader options.
 *
 * @param tsconfig {String} tsconfig.json path.
 */
function tsOptions(tsconfig) {
    var config = require(tsconfig);
    var options = config.compilerOptions;
    options.additionalFiles = config.files.map(function(file) {
        return fp.resolve(fp.join(fp.dirname(tsconfig), file));
    });
    options.instance = "additionalFiles";
    return JSON.stringify(options);
}

[jbrantly]

Added in v0.4.0