Fix: iMap <Cr> Interfere in completion (alias popup-menu, pum) (Issue: #813)

Problem: When pum opened, the mapping of <CR> is triggered but user
want the default behaviour: to choose a completion item

Solution: Conditional mapping:
inoremap <expr><silent><buffer> <CR> pumvisible() ? '<CR>' : ':<Esc>:VimwikiReturn 1 5<Cr>'
                         The previous mapping  -----------------^

test/independent_runs/map.vader

@@ -114,9 +114,11 @@ Do (,w,m -> open tomorrow [Assert]):
 Execute (===========================================================):
   Log "Checking local map"
 
+
 # 2.1 Heading {{{2
 ##############
 
+
 Do (,wn -> Create new wiki [Assert]):
   ,wn
   new_file1
@@ -293,6 +295,24 @@ Expect (Dec header level):
 # 2.2 List {{{2
 ##############
 
+
+
+Given vimwiki (Completion list #813 {{{3):
+  complete1
+  complete2
+  complete3
+
+Do (Insert a list item and complete):
+  Go
+  * comp\<C-n>\<Down>\<Cr>
+
+Expect (With a completion but no new item):
+  complete1
+  complete2
+  complete3
+  * complete2
+
+
 Given (Number list):
   1. I
     1. Relly