respect assignment indentation

This fixes an indentation bug noted by @hadley with e.g. this indentation:

    ```{r, echo = FALSE, message = FALSE}
    ```

      * XYZ

        ```{r}
        a <- 1
    |
   ^->^
        ```

This PR adds a new code path to the `getNextLineIndent` call -- if the 'regular' lookups fail, we fall back to finding the nearest assignment operator, and then using the indentation of the object being assigned to.