emacs: use `use-package` for maintaining packages

Use `use-package` to decentralize the list of installed packages and
keep the package configuration along with the rest of the package
emacs: make imenu index use-package
emacs: require and use rainbow-mode
emacs: use standard function to initialize yas
emacs: initialize buffer-local variables correctly
emacs: don't use customize for backup config
emacs: fix sentence detection

Use single space after period to indicate end of sentence.
emacs: more magit integration
emacs: use company-mode instead of auto-complete
emacs: use project-explorer
emacs: display projectile mode-line in projects

This matches what project-explorer does: fail to work if not in a
project.
emacs: disable popwin

It makes working while having a help buffer open (to read from)
impossible.
emacs: automatically highlight symbols at point