New functionality plus some code reorganization and cleanup

## New Functionality/Behavior

* (Optionally) allow for current line number to be shown in relative number mode. If this is not desired, then user can set 'g:numbertoggle_show_current_line_number_in_relativemode = 0'
* Function to show/hide *all* numbers, regardless of mode (not bound to any key by default, but can easily be bound by, e.g. "nmap <silent> <Leader><F3> <Plug>NumberToggleShowHideAll"
* Documentation update to reflect this changes

## Code Reorganization/Cleanup

* Localized all functions to script namespace
* Localized many variables that need not be in global namespace to script namespace
* Use PLUGIN architecture for flexible and robust specification of key bindings
* `autocmd`'s placed in isolated/protected block