[Improvements] Request headers, filename, rejection callback

[stinodes]

Hi there,

I forked your library for a project and added some (in my case) improvements:

• Component takes a headers-property (needed for authorization headers in my specific case)
• Component takes a fileName-property (I did not have the filename in the URL, and fetching the extension can slower than is desirable)
• Component takes a rejected-component, similar to the placeholder that has already been implemented
• Component takes a onRejected-property that gets called when fetching the image fails.

These additions should be perfectly backwards compatible.

Additional changes I want to implement in my fork, but would not be backwards-compatible:

• Changing the placeholder props and options into a more standard pattern (slots or render-function)

    <CacheableImage
       placeholder={
         <Spinner />
       }/>

or

    <CacheableImage
      placeholder={
        () => <Spinner/>
      }/>

• I was thinking of maybe changing up the code somewhat so the library could be changed to export both a FAC- and a HOC-component, but this would require some larger rearrangements of code. I feel this would make the library more flexible for advanced users. FAC-example

    <CacheableImage
      source="my.resource.com">
      {({pending, source, isRejected}) => 
        pending ? 
          <Spinner/> : 
          <Image source={source}/>
      }
    </CacheableImage>

I can totally make a pull request (or two!) for these, so they can be merged and published under this package, if those changes are something you see fitting here!

Cheers!