Added symmetrical option to disable indents for tab

[amerlyq]

I created pull request based on issue #102 , please, merge it when you pick the mood.

[nathanaelkane]

Thanks for the PR, just a note, you might want to use it with let g:indent_guides_soft_pattern = ' ' as the default soft pattern is \s, which will also match tabs.

i.e.

let g:indent_guides_tab_guides = 0
let g:indent_guides_soft_pattern = ' '