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/base.vim

@@ -1200,7 +1200,7 @@ function! vimwiki#base#follow_link(split, ...)
   else
     if a:0 >= 3
       execute "normal! ".a:3
-    else
+    elseif vimwiki#vars#get_global('create_link')
       call vimwiki#base#normalize_link(0)
     endif
   endif

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',

doc/vimwiki.txt

@@ -2541,6 +2541,18 @@ Value           Description~
 Default: 1
 
 
+------------------------------------------------------------------------------
+*g:vimwiki_create_link*
+
+Create target wiki page if it does not exist. See |:VimwikiFollowLink|.
+
+Value           Description~
+0               Do not create target wiki page.
+1               Create target wiki page.
+
+Default: 1
+
+
 ------------------------------------------------------------------------------
 *VimwikiLinkHandler*