Add ".swx" suffix to file watcher's ignore list

When a user uses Vim to open a file, it briefly creates a
".filename.md.swx" file which needlessly triggers a website rebuild.

According to a comment in Vim's src/memline.c:

    /*
     * on MS-DOS compatible filesystems (e.g. messydos) file.doc.swp
     * and file.doc are the same file. To guess if this problem is
     * present try if file.doc.swx exists. If it does, we set
     * buf->b_shortname and try file_doc.swp (dots replaced by
     * underscores for this file), and try again. If it doesn't we
     * assume that "file.doc.swp" already exists.
     */

commands/hugo.go

@@ -347,7 +347,7 @@ func NewWatcher(port int) error {
 
 				for _, ev := range evs {
 					ext := filepath.Ext(ev.Name)
-					istemp := strings.HasSuffix(ext, "~") || (ext == ".swp") || (ext == ".tmp") || (strings.HasPrefix(ext, ".goutputstream"))
+					istemp := strings.HasSuffix(ext, "~") || (ext == ".swp") || (ext == ".swx") || (ext == ".tmp") || (strings.HasPrefix(ext, ".goutputstream"))
 					if istemp {
 						continue
 					}