feat(emacs/eri.el): add eri.el (from agda mode)
feat(emacs/lean-mode): add lean-tab-indent-or-complete

Close #105
feat(bin/linja): add tags target

fix #117
feat(emacs/lean-tags): call "linja TAGS" to make TAGS

fix #117
fix(bin/linja): use ninja binaries at leanprover.github.io

fix #118
fix(frontends/lean/info_manager): user provided options override saved options, fixes #119

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