Add expansion functions to snippets.

[paulvanbrenk]

Now that we have implemented snippets we should look at expansion functions.. like the C# snippet
<Function>GenerateSwitchCases($expression$)</Function> which allows you to run code when expanding the snippet.

[zpdDG4gta8XKpMCd]

can you make these expansion functions pluggable and customizable? as far as i remember there was no way to write a custom function in C# snippets