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.

autoload/vimwiki/base.vim

@@ -1670,8 +1670,17 @@ function! vimwiki#base#follow_link(split, ...) abort
       let cmd = ':badd '
     elseif a:split ==# 'tab'
       let cmd = ':tabnew '
+    elseif a:split ==# 'tabdrop'
+      " Use tab drop if we've already got the file open in an existing tab
+      let cmd = ':tab drop '
     else
+      " Same as above - doing this by default reduces incidence of multiple
+      " tabs with the same file.  We default to :e just in case :drop doesn't
+      " exist in the current build.
       let cmd = ':e '
+      if exists(':drop')
+        let cmd = ':drop '
+      endif
     endif
 
     " if we want to and can reuse a split window, jump to that window and open
@@ -1684,7 +1693,6 @@ function! vimwiki#base#follow_link(split, ...) abort
       endif
     endif
 
-
     if vimwiki#vars#get_wikilocal('syntax') ==# 'markdown'
       let processed_by_markdown_reflink = vimwiki#markdown_base#open_reflink(lnk)
       if processed_by_markdown_reflink