Add ability to jump to next offsetted indent

I really like the concept of jumping through the file by using the indent. Today I thought it would be nice if you could also move to the next line which is "more/less indented" than the current line.

Small example:
```
def aFunction():
  # a lot code
  # ...
  # code [cursorPosition]
```

If I want to move to the function head, I would usually move the cursor to the beginning of the line and invoke ```jump_prev_indent```. A better way would be to simply invoke a shortcut which jumps to the line which has the next smaller indent.

I implemented it so that you can pass an ```indent_offset``` parameter to ```jump_next/prev_indent```. The offset would be normally 1 or -1 (for an indented and unindented offset). The default is zero, so that the default behavior doesn't change.

I kept the new tests rather minimal, since the basic functionality is already thoroughly tested in the existing tests. I hope, this is ok.