Feature: Config: Normalize link so that user can choose [[]] or []() (Issue #892)

Main change:
        Create function: normalize_syntax_settings(syntax)

Problem:
        Some user prefer [[]] links more than []().
        As vimwiki know both of them, they should be able to choose
        without regex mastery and with tests

Solution:
        let g:vimwiki_syntax_variables.markdown.Link1 = vimwiki#vars#get_global('WikiLinkTemplate1')

test/link_creation.vader

@@ -1,5 +1,30 @@
 Include: vader_includes/vader_setup.vader
 
+# Link Normalisation {{{1
+# And configuration
+# Issues: #892
+####################
+
+Execute (Log):
+  Log 'Markdown change Link1 : Pressing enter to create a [[double bracket]] #892'
+
+Given vimwiki (abc def ghi jkl):
+  abc def ghi jkl
+
+Execute (Set filename wiki_test.md):
+  call SetSyntax('markdown')
+  let save_link = g:vimwiki_syntax_variables.markdown.Link1
+  let g:vimwiki_syntax_variables.markdown.Link1 = vimwiki#vars#get_global('WikiLinkTemplate1')
+
+Do (vee<CR>):
+  vee\<CR>
+
+Expect (append md suffix):
+  [[abc def]] ghi jkl
+
+Execute (restore):
+  let g:vimwiki_syntax_variables.markdown.Link1 = save_link
+
 
 # vimwiki_markdown_link_ext {{{1
 ####################