Don't alias mvim because symlink named vim to mvim

The `mvim` script with MacVim is smart to check for the name it was called with. If the name is "vim" then it launches the command line version of MacVim by adding `-v` to the MacVim binary launcher. So create a symlink called "vim" that points to "mvim" instead of relying on an alias which wouldn't for scripts that call `vim` externally - e.g. git.