Add SSI include file=, include virtual=

test/data/ssi/f1.txt

@@ -0,0 +1 @@
+this is f1

test/data/ssi/index.shtml

@@ -0,0 +1,3 @@
+this is index
+<!--#include file="nested.shtml" -->
+<!--#include file="recurse.shtml" -->

test/data/ssi/nested.shtml

@@ -0,0 +1,3 @@
+this is nested
+<!--#include virtual="/notexist.txt"-->
+<!--#include virtual="/ssi/f1.txt"-->

test/data/ssi/recurse.shtml

@@ -0,0 +1,2 @@
+<!--#include file="recurse.shtml"-->
+recurse

test/unit_test.c

@@ -349,7 +349,7 @@ static void eh1(struct mg_connection *c, int ev, void *ev_data, void *fn_data) {
       struct mg_http_serve_opts opts = {".", NULL};
       mg_http_serve_dir(c, hm, &opts);
     } else {
-      struct mg_http_serve_opts opts = {"./test/data", NULL};
+      struct mg_http_serve_opts opts = {"./test/data", "#.shtml"};
       mg_http_serve_dir(c, hm, &opts);
     }
   } else if (ev == MG_EV_WS_MSG) {
@@ -477,6 +477,17 @@ static void test_http_server(void) {
                "Content-Length: 4\r\n\r\nkuku") == 200);
   ASSERT(cmpbody(buf, "kuku") == 0);
 
+  ASSERT(fetch(&mgr, buf, url, "GET /ssi HTTP/1.1\r\n\r\n") == 200);
+  ASSERT(cmpbody(buf,
+                 "this is index\n"
+                 "this is nested\n\n"
+                 "this is f1\n\n\n\n"
+                 "recurse\n\n"
+                 "recurse\n\n"
+                 "recurse\n\n"
+                 "recurse\n\n"
+                 "recurse\n\n") == 0);
+
   ASSERT(fetch(&mgr, buf, url, "GET /badroot HTTP/1.0\r\n\n") == 400);
 #if MG_ARCH == MG_ARCH_WIN32
   ASSERT(cmpbody(buf, "Bad web root [Z:\\BAAADDD!]\n") == 0);