Scaladoc: Roadmap of Snippet Compiler

[pikinier20]

Proposals for further development of Snippet Compiler inside Scaladoc:

1. Add a way to hide pieces of code inside snippet with possibility to reveal it.
2. Add flags to indicate if snippet should or shouldn't compile. If the snippet shouldn't compile we should attach compilation errors to documentation.
3. Add code markers (just like markers showing potentially wrong code in compilation errors) and assert that marked piece of code doesn't compile. There's also interesting idea to implement nice UI to code snippets which highlight marked code and show error on hover.
4. Try to backport Snippet Compiler to Scala 2.

[pikinier20]

@romanowski I feel like I missed some ideas but can't recall anything else

[romanowski]

Looks good.