Bump version to 1.21 to reflect addition of new symbols

This covers both exponentiation (`**`) and `generate`/`endgenerate`

README.md

-## iStyle 1.20 (Fast and Free Automatic Formatter for Verilog Source Code) ##
+## iStyle 1.21 (Fast and Free Automatic Formatter for Verilog Source Code) ##
 > (Created by haimag, Report Bugs: haimag@gmail.com)
 > (Thanks to Tal Davidson & Astyle)
 > Originally hosted at http://code.google.com/p/istyle-verilog-formatter before Google Code EOL

src/astyle_main.cpp

@@ -57,7 +57,7 @@ using namespace astyle;
 ostream *_err = &cerr;
 string _suffix = ".orig";
 
-const string _version = "1.20";
+const string _version = "1.21";
 bool shouldBackupFile = true;
 
 // --------------------------------------------------------------------------