Fix bug with min_indent and max_indent settings

Fixes https://github.com/roryokane/detectindent/issues/6

> The code that handles `g:detectindent_min_indent` takes the maximum of `g:detectindent_min_indent` and `&l:shiftwidth`, when it should be comparing against `shiftwidth()`. See `:help shiftwidth()` for the difference. This is is relevant now that I set the `'shiftwidth'` option to 0 by default.

For now, I have chosen to save the calculated indent-width to a variable and refer to it there. After extracting RestrictIndentWidthWithinOptionRange, I might want to refactor this by instead defining a shim for `shiftwidth()` and using it there.