feat(bin/linja): add --discard and --to_axiom options
feat(util/lean_path.cpp): use '/library' as LEAN_PATH for emscripten
feat(shell/lean.cpp): expose functions to javascript side

lean_init, lean_import_module, lean_process_file
feat(CMakeLists.txt): support emscripten
fix(emacs/lean-util): fix lean-path-list to only include existing dirs

Fix #246
fix(style): remove "using namespace emscripten"
refactor(library): move flycheck "helper" classes to separate module