Autocompletion for VimwikiGoto command.

Adding auto-completion for `:VimwikiGoto` command.

I'll be adding more autocompletion stuff when I stumble upon specific use-cases.

Let me know if you need me to add support for more (specific) commands right away, if that's required for a merge.

Note: this version is 'simplified', as it does not interpret arg#2 of `:VimwikiGoto` properly.  Arg#2 is supposed to be anchor (at least, internally, the support is there even though not advertised in help page).  But this function will suggest completion of 2nd arg as if it was another link.