emacs: fix enabling yasnippet

need to provide argument to yas-global-mode call otherwise it complains
emacs: replace dired+ with plain dired

I think I can deal with dired opening folders in new buffers now that I
don't care as much about having loads of buffers open (using ibuffer and
ido helps with that).

But by enabling the command dired-find-alternate-file, pressing 'a'
instead of return on a folder or file will replace the current dired
buffer with the selected file or folder. So I can get that same
behaviour if I want to.
emacs: go back to using tango-dark theme

It's not perfect but it's the least bad overall (most others I tried
were better in some modes but much worse in others.