.vimrc: Set soft tab stop to shiftwidth

For more consistent behavior with tab, indent, and delete when pre-
ferring spaces over tabs.

See: http://vimcasts.org/episodes/tabs-and-spaces/

.vimrc

@@ -1,5 +1,6 @@
 set number          " show line numbers
 set tabstop=4       " number of visual spaces per tab
+set softtabstop=4   " number of spaces in tab when editing
 set shiftwidth=4    " number of spaces in an indent
 set expandtab       " tabs are spaces
 set backspace=2     " backspace like in a graphical text editor