Default options "constructor" and fileExtension option

[d1manson]

As it says in the title, I've added two new things here.

The new docs in the readme and extra tests hopefully give sufficient explanation, but essentially I wanted an easy way to specify the file extension at the top of the test file and then have lots of .toMatchFile assertions that automatically use that file extension. Note that this isn't a breaking change.

This didn't take that long to do, so I am happy to make some slight tweaks if needed, but obviously I'd be keen to get it merged with minimal effort if possible.

Thanks!

[d1manson]

Initially I thought you could scope your extension to a describe/it block, but that doesn't seem to be the case. I've had a quick search of the docs to confirm that extensions are at least scoped to a file but couldn't actually find that mentioned (though I'm fairly sure it is the case).
At any rate, I've used separate test files in order to test out multiple versions of the extension.

[satya164]

Thanks for the PR.

I'm not sure another matcher toMatchFileUsing is necessary. Can't we just do the following:

expect.extend({
  toMatchFile: (content, filepath, options) =>
    toMatchFile(content, filepath, { fileExtension: '.md', ...options }),
});

I just merged #29 which adds a fileExtension option.