Add helpers for finding starting line of a node

The Sass parser is lossy in that it does not store offsets/ranges of the
parse tree nodes it produces. For any node, it will always report a line
corresponding to the last line the node spans. For example, the
following rule set will have its `line` set to line 3:

    1  .classA,
    2  .classB,
    3  .classC {
    4    ...
    5  }

There are cases where we need to know the starting line of a node in
order to check for certain lints. Add a helper `node_start_line` which
can deduce the starting line of a node.

In order to support this, we need to monkey patch `Sass::Tree::Node`s
and `Sass::Script::Node`s to have a `node_parent` attribute which we can
write to so that nodes can be aware of the nodes that contain them. We
then change the implementation of `visit_children` of the `Linter`
(which inherits its implementation from `Sass::Tree::Visitors::Base`) to
set the `node_parent` before visiting each node.

Change-Id: I6d5eeca42bb38495c883ff886518178bdea5a4ec
Reviewed-on: https://gerrit.causes.com/29759
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>