feat(bin/lmake) add --just-print (-n) option
feat(library/Makefile.common): add tags target
fix(bin/lmake): use "--always-make" option instead of touch

close #42
feat(bin/ltags): add ltags

Usage:

    ltags <ilean_files> # generate TAGS file for <ilean_files>

    ltags  # Find makefile upwards, generate TAGS file for all ilean files

close #50
chore(library/.gitignore): add TAGS
feat(emacs): add lean-find-tag (M-.)
feat(emacs): add lean-complete-tag (tab)