feat: add skipImport option

[gutenye]

To use with libs other than React, for example Preact.

import svgr from '@svgr/core'
const result = await svgr(code, { skipImport: true })
return `
  import { h } from 'preact'
  ${result}
`

This is more flexiable.

[vercel]

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[gregberge]

Fixed in #613