Add Webpack 4 / ts-loader 4 / awesome-typescript-loader 4/5 compatibility

[strothj]

I plan to support the changes required for Webpack 4. I'm currently waiting for these:

storybookjs/storybook#3148
storybookjs/storybook#3044

[strothj]

From my early testing, the loader seems to work without modification with Webpack 4. I'm going to leave this issue open until Storybook is updated to support Webpack 4 so that tests can be added.

[strothj]

Closing since tests now verify against Webpack 4, ref: f5d1d46.