Add option to ignore current line when trimming whitespace

It's entirely possible that there's a better way to do this, but I came up with this solution. This adds a new `ignoreWhitespaceOnCurrentLine` option. When enabled, it prevents saves from trimming whitespace on the current line. This is to catch situations where you've just started a new method definition, have the leading indentation in place, and then save. You obviously don't want that whitespace to disappear. Or situations where you're typing a Markdown list, start a new bullet point with a space, and then save. It works with multiple cursors too.

Depends on atom/text-buffer#4 being merged to get `TextBuffer#backwardsScan`.