support include: 'initial'

[yyx990803]

This allows us to use the following setup:

plugins: [
  new HTMLWebpackPlugin(),
  new PreloadPlugin({
    // rel: 'preload'
    include: 'initial'
  }),
  new PreloadPlugin({
    rel: 'prefetch'
    // include: 'asyncChunks'
  })
]

[jeffposnick]

Thanks, @yyx990803!

I brought up something similar at #40, but the approach there is effectively a breaking change, compared to the incremental change in this PR.

I'd be open to merging this and tagging it v2.2.0, and then potentially going with an approach described in #40 for 3.0.0.

CC: @addyosmani to give him a chance to comment about that plan if he'd like. Otherwise, I'll merge.

[jeffposnick]

I've published this as 2.2.0 on npm.