New `g:indent_guides_soft_pattern` option

I added a new option to explicitly specify a pattern for indentation. For example it allows to use spaces for alignment by matching them in the beginning of line and ignore them after tabs. The new option doesn't brake anything and back compatible with older versions of the plugin. Check out the screenshot.

![screenshot](https://raw.github.com/gist/3872070/c90c6b6af0fef85a845075df932977384f0db319/screen.png)