Add g:vimwiki_create_link option to prevent link creation.

Adds a new global option named create_link, which allows the user to
control :VimwikiFollowLink behavior when attempting to follow a
non-existent link. The original behavior of creating a new link is
preserved. However, by setting this option to 0, new links will not be
created when pressing return over regular text. Closes #528.

autoload/vimwiki/vars.vim

@@ -145,6 +145,7 @@ function! s:read_global_settings_from_user()
         \ 'auto_chdir': {'type': type(0), 'default': 0, 'min': 0, 'max': 1},
         \ 'autowriteall': {'type': type(0), 'default': 1, 'min': 0, 'max': 1},
         \ 'conceallevel': {'type': type(0), 'default': 2, 'min': 0, 'max': 3},
+        \ 'create_link': {'type': type(0), 'default': 1, 'min':0, 'max': 1},
         \ 'diary_months': {'type': type({}), 'default':
         \   {
         \     1: 'January', 2: 'February', 3: 'March',