P Sz

Fix source indices for Sass interpolated selectors

2

Fixes https://github.com/postcss/postcss-selector-parser/issues/243 I had to rewrite most of the `splitWords` parser method because the previous one didn't use the correct indices when multiple tokens were involved.

Repro: ``` test('Sass selector', '.#{$classname}', (t, tree) => { const node = tree.nodes[0].nodes[0]; t.deepEqual(node.type, "class"); // Ok t.deepEqual(node.value, "#{$classname}"); // Ok t.deepEqual(node.source.start.column, 1); // Fail. Received: 5 t.deepEqual(node.source.end.column, 14); //...