Added `context` argument to the `processString` method

This is a **work in progress**, it is not complete yet, but I'm submitting this, 'cause I'm not sure on the naming stuff etc, so you could review early and suggest any changes. I'll then fix them and rebase the branch.

The point of this PR is to allow combing of the incomplete input, so we could then use CSSComb.js in more contexts, like this one — #177 (combing of the selected blocks or declarations only).