Improvements to trailing whitespace handling

Applies two changes:
1.  wraps autocmd for removing trailing whitespaces in an augroup block, as recommended by @sjl http://learnvimscriptthehardway.stevelosh.com/chapters/14.html#using-autocommands-in-your-vimrc
2.  Add a command `:WWS` to write while suspending autocmd, which allows the writing of a file without removing trailing whitespace - see #64