Added option to disable unicode in vimrc.

Hi Peter,

I created this patch because I wanted to have plain ASCII bullets in my notes.
I first tried setting g:notes_list_bullets to just the ASCII version but since
the '*' character was being automatically converted to the unicode bullet,
Tab/Alt-tab wasn't working. I also tried setting g:notes_smart_quotes to 0 but 
then new bullets weren't being inserted when hitting return. This seemed like
the simplest way to get my desired behavior.

The new option is: g:notes_unicode_enabled

Thanks for working on this plugin.

Jon