Allow tabs for indentation

[prez]

Currently, attempting to indent key = value pairs with tabs causes expected name errors, which is counter-intuitive. Especially considering that tabs are valid whitespace for line continuation.

The Ninja devs seem to be ignoring this (ninja-build/ninja#1598).

If there is a good reason to reject this, please consider raising an appropriate error message.

Thank you.

[Michael Forney]

Yes, I think that would be quite nice to have, and I see no reason why allowing them would be a problem. Although scripts that use tabs would be incompatible with ninja.

This should be an easy change.