Configure signcolumn to automatically toggle

Vim 8 adds a new option where the sign column can be always visible
(yes), always hidden (no) or automatically toggled (auto) when signs are
available to display. Prefer the more generic option over the plugin
specific one, and set to automatically toggle.

More details:
arcticicestudio/igloo#32
airblade/vim-gitgutter@dc73a81
airblade/vim-gitgutter#431 (comment)

vimrc.local

@@ -26,7 +26,7 @@ let g:airline_symbols.branch = ''
 let g:airline_symbols.readonly = ''
 let g:airline_symbols.linenr = ''
 
-let g:gitgutter_sign_column_always = 1
+set signcolumn=auto
 
 let g:vimwiki_list = [{'path': '~/.vimwiki',
                      \ 'syntax': 'markdown', 'ext': '.md'}]