deassociate COMMIT_EDITMSG from auto-complete mode

I installed git-commit-mode from MELPA, which offers 50 char title
warning, 72 char fill-columne of body, and enhanced coloring of
COMMIT_EDITMSG.
restore fundamental mode to default

Fundamental mode --> text mode was the residual effect of an awkward way
to autofill COMMIT_EDITMSG. Now it can be discarded.
auto indent with RET in CC, TeX, and Python modes

Now distributed into the block of each mode rather than centralized at
the beginning.
minor changes for consistency of style
auto-fill-mode for CC, TeX, and Python modes

Also reset the fill-column to default (70). 72 was for COMMIT_EDITMSG.
stop loading custom packages from .emacs.d/custom

No custom package to load now. All packages are managed by ELPA and
MELPA.
remove markdown mode setup code

`markdown-mode` is now managed by MELPA.
upgrade .emacs format

Highlights:
1. Add blank lines for clarity;
2. Split into "Global" and (major) modes.