Allow EditorConfig not touching softtabstop via [] config

Picking up the argument from @chreekat in #137, it can be argued that
'softtabstop' purely is a user experience feature and the EditorConfig plugin
should not modify it at all. (This argument can be made since Vim 7.3.693, where
a negative value makes 'softtabstop' automatically assume the value of
'shiftwidth'; a value of 0 disables the feature altogether. However, I think
most users would rather see 'softtabstop' as one of the indent options,
especially when positive values are assigned to it.)

A special (empty) List configuration value can make the EditorConfig plugin skip
the update of 'softtabstop'.

doc/editorconfig.txt

@@ -167,14 +167,16 @@ backspace key delete one indent level. If you turn off that feature (by
 setting the option to 0), only a single space will be deleted.
 This option defaults to 1, which enables 'softtabstop' and uses the
 'shiftwidth' value for it. You can also set this to -1 to automatically follow
-the current 'shiftwidth' value (since Vim 7.3.693).
+the current 'shiftwidth' value (since Vim 7.3.693). Or set this to [] if
+EditorConfig should not touch 'softtabstop' at all.
 
                                             *g:EditorConfig_softtabstop_tab*
 When tabs are used for indent, Vim's 'softtabstop' feature only applies to
 backspacing over existing runs of spaces.
 This option defaults to 1, so backspace will delete one indent level worth of
 spaces; -1 does the same but automatically follows the current 'shiftwidth'
 value. Set this to 0 to have backspace delete just a single space character.
+Or set this to [] if EditorConfig should not touch 'softtabstop' at all.
 
                                             *g:EditorConfig_verbose*
 Set this to 1 if you want debug info printed:

plugin/editorconfig.vim

@@ -402,14 +402,18 @@ function! s:ApplyConfig(config) abort " Set the buffer options {{{1
         " value
         if a:config["indent_size"] == "tab"
             let &l:shiftwidth = &l:tabstop
-            let &l:softtabstop = g:EditorConfig_softtabstop_tab > 0 ?
-                        \ &l:shiftwidth : g:EditorConfig_softtabstop_tab
+            if type(g:EditorConfig_softtabstop_tab) != type([])
+                let &l:softtabstop = g:EditorConfig_softtabstop_tab > 0 ?
+                            \ &l:shiftwidth : g:EditorConfig_softtabstop_tab
+            endif
         else
             let l:indent_size = str2nr(a:config["indent_size"])
             if l:indent_size > 0
                 let &l:shiftwidth = l:indent_size
-                let &l:softtabstop = g:EditorConfig_softtabstop_space > 0 ?
-                        \ &l:shiftwidth : g:EditorConfig_softtabstop_space
+                if type(g:EditorConfig_softtabstop_space) != type([])
+                    let &l:softtabstop = g:EditorConfig_softtabstop_space > 0 ?
+                            \ &l:shiftwidth : g:EditorConfig_softtabstop_space
+                endif
             endif
         endif