follow_link: reuse existing tabs with tab drop (closes #238)

Replaces :e in `vimwiki#base#follow_link()` with :drop, making this the
default behavior for pressing <CR> on a link.  Checks for the existence
of :drop first, since this isn't available in some builds.

Adds a new :VimwikiTabDropLink and makes this a default for the keybindings
formerly occupied by :VimwikiTabnewLink; leaving :VimwikiTabnewLink
available for backwards compatibility and anyone who still wants the old
behavior.

Doesn't touch the split window reuse functionality, or the
:VimwikiGoBackLink behavior, although I can see an argument for adding
:drop to the latter.

I've wanted this for a while and happened to notice @davidlmontgomery's
patch from 2016 in #238.  Code has moved around a little since, but I
think this is correct.

doc/vimwiki.txt

@@ -370,8 +370,10 @@ MAP                MODE
 <C-S-CR>, <D-CR>   n    Follow wiki link (create target wiki page if needed),
                         opening in a new tab.
                         May not work in some terminals. Remapping could help.
-                        Maps to |:VimwikiTabnewLink|.
-                        Remap command: `<Plug>VimwikiTabnewLink`
+                        Maps to |:VimwikiTabDropLink|.
+                        Remap command: `<Plug>VimwikiTabDropLink`
+                        See |:VimwikiTabnewLink| for old behavior which
+                        always opens a new tab.
 
                                                          *vimwiki_<Backspace>*
 <Backspace>        n    Go back to previously visited wiki page.
@@ -810,6 +812,11 @@ Vimwiki file.
 *:VimwikiTabnewLink*
     Follow wiki link in a new tab (create target wiki page if needed).
 
+*:VimwikiTabDropLink*
+    Follow wiki link with tab drop, reusing any existing tabs with the page.
+    Creates a new tab if the page isn't already open (create target wiki page
+    if needed).
+
 *:VimwikiNextLink*
     Find next link on the current page.
 
@@ -3985,6 +3992,7 @@ http://code.google.com/p/vimwiki/issues/list. They may be accessible from
 https://github.com/vimwiki-backup/vimwiki/issues.
 
 New:~
+    * Feature: #238: Reuse existing tabs with tab drop
     * Issue #621: Feature request: Highlight code listings in HTML
     * Issue #290: Calendar plugin, do not sign if no wiki
     * Issue #281: Permit `\|` in tables