Add option for default sizes & setting output context

[alex-ketch]

Thanks for creating this loader. Let me know if you’d be open to the following PR.

This PR adds ability to declare a default Sizes array in webpack.config.js, and uses those sizes in case neither sizes nor size are declared in the query.

Default name, context, quality, and background options can now also be defined.

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Additionally this interprets the context variable, if present in the query or settings.
This allows flexibility when outputting files.

For example my Webpack project structure is like this:

|-- app <-- source files
|----assets
|------images
|--------someFile.jpg
|-- dist <-- webpack output destination

Now I can structure my query to be loader: 'responsive?name=/[path][name][hash]-[width].&context=./app/' to output files to dist while keeping the rest of the nested folder structure from app/assets.

|-- app <-- source files
|----assets
|------images
|--------someFile.jpg
|-- dist <-- webpack output destination
|----assets
|------images
|--------someFile[someHash]-[width].jpg

[jstcki]

Nice! I didn't know about context but apparently file-loader uses it too. Do you want to include #13 in this PR too?

[alex-ketch]

Yeah, I think context is very poorly documented—if it is at all. Came across it by accident while trying to tidy up the output folder structure.

As for #13, most likely it will be a few days before I can get to it it. So maybe it's better I make a separate PR for that?

Thanks again for taking a look at this