Support val and var statements

This is an attempt to implemet #10 (supporting `val` and `var` statements). I've renamed the `Import` case of `DoctestComponent` to `VerbatimLine` and use it to also support assignments. That means that `val`s are in the same scope as the imports (inside the `describe` but not inside the `it`). So you can't use two `val`s with the same name in one Scaladoc. This would yield a compile error:
```scala
/**
 * {{{
 * >>> val x = 1
 * ...
 * >>> val x = 2
 * }}}
 */
```