feat(emacs/lean-mmm-lua.el): add mmm-mode to syntax-highlight lua code
in lean-mode

Close #74
feat(emacs/lean-server): accept "BEGININFO .*" pattern
feat(frontends/lean/server): add 'FINDP' command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(emacs/lean-mode): Bind TAB with lean-tab-indent-or-compliete
checkpoint for imenu