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.

This also implies that indentation rules now operate like:

```R
x <- 1
    y <- 2
    |
```

rather than aligning to the first column.