Optipng (#156)

* omg it’s compiling

* example actually works

* Expose compression level options

* Disable crypto and path module emulation in webpack

* Update README

* Remove small image

* Use -O3 on optipng

* Free memory after copy

* Handle unexpected file reader return types

* Rename level label to effort

codecs/optipng/optipng.d.ts

@@ -0,0 +1,10 @@
+import {EncodeOptions} from "src/codecs/optipng/encoder";
+
+export interface OptiPngModule extends EmscriptenWasm.Module {
+  compress(data: BufferSource, opts: EncodeOptions): Uint8Array;
+  free_result(): void;
+}
+
+export default function(opts: EmscriptenWasm.ModuleOpts): OptiPngModule;
+
+