cdlatex is not packaged in nix yet, turn off for now (coroa)
fix push-due-to-0x2c function to update blocks before export (coroa)
map nix mode if available

unfortunately, auto loading and conditional loading (only if
nix-mode.el is available) is incompatible. or more difficult, low prio
task for the future.
fix ido-imenu which stopped working in emacs 24.3.1

flet has become deprecated and is now just a synonym for cl-flet,
which does not allow recursive function definitions. cl-labels does.