Need `includePattern` as option

My issue is that we want to use this babel transform just for icons that we want to style - inline icons that should have the same color as surrounding text, etc.

On the other hand, we'd like to use webpack's `url-loader` to handle all larger "illustration" SVGs. We would rather put those in `img` tags and let them be cached by the browser and not have them in our javascript bundle.

To do so, an `includePattern` could be used that would only use this babel transform on (for example):
- `*.icon.svg`
- `*/icons/*.svg`

Thoughts? I'm about to make a PR.

Start by reviewing the plugin's current option handling and how it selects SVG imports. Compare the requested *.icon.svg and */icons/*.svg patterns with existing behavior. Done means the transform applies only to matching files while other SVGs remain available to webpack's url-loader, with coverage for the new option.

Written by the indexing model from the issue text.