feat: support url-loader & file-loader

[gregberge]

Following facebook/create-react-app#1388 (comment)

When you use url-loader + svgr/webpack or file-loader + svgr/webpack:

import { ReactComponent }, url from './file.svg'

When you use only svgr/webpack:

import ReactComponent from './file.svg'

[iansu]

This will work for the create-react-app use case. Thanks!

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.46%.
The diff coverage is 42.1%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage    39.8%   40.27%   +0.46%     
==========================================
  Files          22       22              
  Lines         206      221      +15     
  Branches       49       52       +3     
==========================================
+ Hits           82       89       +7     
- Misses         96      102       +6     
- Partials       28       30       +2

Impacted Files	Coverage Δ
src/index.js	100% <ø> (ø)	⬆️
src/webpack.js	0% <0%> (ø)	⬆️
src/transforms/wrapIntoComponent.js	80% <100%> (+17.5%)	⬆️

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