Add zig support, closes #405

ftplugin/zig.vim

@@ -0,0 +1,18 @@
+if exists('g:polyglot_disabled') && index(g:polyglot_disabled, 'zig') != -1
+  finish
+endif
+
+" Only do this when not done yet for this buffer
+if (exists("b:did_ftplugin"))
+  finish
+endif
+
+let b:did_ftplugin = 1
+
+set expandtab
+set tabstop=4
+set shiftwidth=4
+
+setlocal suffixesadd=.zig
+setlocal commentstring=//\ %s
+setlocal makeprg=zig\ build