Support reading zstd files

Similar to how we already support gzip files, allow reading zstd files
from disk, under their non zstd name.  Files ending in .zstd will now be
treated as zstd compressed.

Tested:
Manually loaded a zstd core dump into the folder;  Curl succeeds in
getting file.

Change-Id: I49a92b823117f89fcaebd3b54f0ad93582b3bbdf
Signed-off-by: Ed Tanous <ed@tanous.net>
diff --git a/include/webassets.hpp b/include/webassets.hpp
index 9ac2abd..d465ca0 100644
--- a/include/webassets.hpp
+++ b/include/webassets.hpp
@@ -166,6 +166,13 @@
         extension = webpath.extension().string();
         file.contentEncoding = "gzip";
     }
+    else if (extension == ".zstd")
+    {
+        webpath = webpath.replace_extension("");
+        // Use the non-zstd version for determining content type
+        extension = webpath.extension().string();
+        file.contentEncoding = "zstd";
+    }
 
     file.etag = getStaticEtag(webpath);