Added tag support for code

Tags defined in the options when initializing the code object can now be used instead of indices.

Example:
```
var code = av.code("line 1\n" + "line 2\n" + "line 3", {tags: {line1: 0, line2: 1, line3: 2 }});

code.setCurrentLine("line1");
code.setCurrentLine("line2");
code.setCurrentLine("line3");
code.highlight("line1");
```