Better autodetection of Soft Tabs.

Check for the presence of a space, instead of the absence of a tab.
This ensures that Soft Tabs will not be activated on a file that (for
example) starts with a BOM or something, but is otherwise full of hard
tabs.

This issue is discussed on the Atom discussion board at:
https://discuss.atom.io/t/tabs-still-dont-work-package-related
Should this regex just use `[ \t]` instead so it only matches regular spaces and tabs and not a BOM?