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 `ignoreLeadingWhitespaceOnCurrentLine` option. When enabled, it prevents saves from trimming whitespace on the current line – if that line only contains whitespace. 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.

The only major caveat here is that I haven't played around much with what happens if multiple cursors exist. It probably won't work as expected as is.