added sub_str

docs/lib-sub_str.md

@@ -0,0 +1,14 @@
+# sub_str
+
+Returns a new string that is a substring of the given string.
+
+## Parameters
+
+- `t` : The original string.
+- `begin` : The beginning index, inclusive.
+- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of this string.
+
+## Examples
+
+	echo(sub_str("helloworld", 0, 5)); // ECHO: "hello"
+	echo(sub_str("helloworld", 5));    // ECHO: "world"