Adds a new backend, and fixes a serious Emscripten bug.

• Adds IsoFS backend. Mount ISO files in the browser! Supports Joliet and Rock Ridge extensions to ISO9660 standard, as well as vanilla ISO9660 files.
• '.' and '..' now show up in Emscripten directory listings. Previously, these were missing from readdir calls, which is consistent with Node behavior and inconsistent with Emscripten behavior.