Rename .vimrc placeholder file, add feature list.

Also add attributions for sources of .vimrc snippets.
Minor typo: nix.vimrc -> nix.vim
Second minor typo: nix.vim -> nix_vimrc.vim.

D'oh!
First hack at .vimrc file.

Includes all of the features mentioned in the README.md file. All
in one commit because I want a text editor to start with a good
base feature set when I work with it; I don't want to incrementally
add changes and have to enable things like syntax highlighting, and
search highlighting, and so forth, over and over and over. This set
of features is a good start.