Allow optional custom svgmin plugins

[tannerhodges]

Got another custom option to add for svgmin plugins.

Realized that my build needed some different options, so I just tacked on another argument:

mix.svgstore('resources/assets/icons/global', 'public/sprites/', 'global.svg', [
  { removeUnknownsAndDefaults: false },
  { removeUselessStrokeAndFill: false },
  { collapseGroups: false }
]);

When left blank, it uses the original default plugins you had set.

Ideally, I'd clean this up a bit by using the Elixir config to handle settings like this. But for now, this gets the job done.

Oh, and in case anybody wanted to set custom plugins but still use the default source/output/filename, you can just pass null for those first couple of arguments. Like so:

mix.svgstore(null, null, null, [
  { removeUnknownsAndDefaults: false },
  { removeUselessStrokeAndFill: false },
  { collapseGroups: false }
]);

Again, this could be cleaned up but wanted to get this first pass in.

[sknoslo]

Sorry for the delay on this. This will be a very useful feature. I think the defaults are useful though... so could you perhaps update the README to document what the default options are set to?

That way users wont accidentally override the defaults when they didn't mean to.

I'll happily accept this PR when that is done. Thanks!

[tannerhodges]

@sknoslo No worries. Added a note to the "By default..." section of the readme. How's that look?

[sknoslo]

Looks good to me. Thanks!