.vimrc: Set solarized transparency option

When solarized sets a background color it causes text selection via
Xorg/mouse to include otherwise blank space as actual spaces when
you paste it.  For some reason this only happens when using vim
inside screen/tmux.  Setting the transparency option solves it.

.vimrc

@@ -15,4 +15,5 @@ if version >= 600
 endif
 
 let g:solarized_termcolors=256
+let g:solarized_termtrans=1
 colorscheme solarized