feat(emacs/lean-cmd): add lean-clear-cache

Close #99
feat(bin/linja): add clear-cache target

Close #99
feat(bin/linja): return exitcode from ninja and lean

fix #120
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>
feat(frontends/lean): add 'class' command back

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>