fix(frontends/lean): bug in include/omit commands: in the end of section/context, the configuration must be restored
refactor(frontends/lean): remove 'including' expressions
fix(emacs/lean-changes): visit file before process any changes

This solves many instances of the modified buffer problem.
feat(emacs/lean-server): delete cache file (.clean) after visit/load

fix #75
feat(emacs/lean-mode): add 'clean-cache' to the menu

Related issue: #75
feat(emacs/lean-server): send SYNC command when visiting a modified buffer

Close #201