Add ability to override the Builder XML indent amount

Hello,

this change allows to pass the `:indent` option to to Builder.

It also works well inside @Sinatra, as `set :builder, :indent => 0` will pass that options hash to Tilt that will pass it along to Builder.

In the [1.3.7-indent branch](https://github.com/ifad/tilt/tree/1.3.7-indent) there's also a cherry-pick to `1.3.7`.

Thanks,

~Marcello