Significant plugin overhaul

Several major updates to the plugin overall:
1.    Made highlighting behave more sanely in all modes
2.    Provided several helper commands for users ( `:ToggleWhitespace`, `:DisableWhitespace`, `:EnableWhitespace` )
3.    Also provide options to disable current line highlighting in normal mode ( `:CurrentLineWhitespaceOff <level>`, `:CurrentLineWhitespaceOn` )
4.    An option to fix whitespace on save ( `:ToggleFixWhitespaceOnSave` )
5.    Now allows users to set a custom highlight color for whitespace that won't be overridden by this plugin:
        `highlight ExtraWhitespace ctermbg=<desired_color>`