Accept MySQL dbname beginning with digits, fix #230.

pgloader used to have a single database name parsing rule that is
supposed to be compliant with PostgreSQL identifier rules. Of course it
turns out that MySQL naming rules are different, so adjust the parser so
that the following connection string is accepted:

  mysql://root@localhost/3scale_system_development

src/parsers/command-mysql.lisp

@@ -101,10 +101,17 @@
 
 (defrule mysql-prefix "mysql://" (:constant (list :type :mysql)))
 
+(defrule mysql-dsn-dbname (and "/" (* (or (alpha-char-p character)
+                                          (digit-char-p character)
+                                          punct)))
+  (:destructure (slash dbname)
+		(declare (ignore slash))
+		(list :dbname (text dbname))))
+
 (defrule mysql-uri (and mysql-prefix
                         (? dsn-user-password)
                         (? dsn-hostname)
-                        dsn-dbname)
+                        mysql-dsn-dbname)
   (:lambda (uri)
     (destructuring-bind (&key type
                               user