unmap a binding or delete a command when expression is empty
unmap a binding or delete a command when expression is empty

With this commit `map`/`cmd` keywords are also used to remove a
binding/command by leaving the expression part empty (e.g. `map j`
removes keybinding for `j` or `cmd trash` removes `trash` command).

Mentioned in #16.

gokcehan/lf,gokcehan/lf