Document how to set properties

I got a bit confused because someone in our code base had written the below and it was failing to actually use classes.

ansi_up.ansi_to_html('txt', {
  use_classes: true
})

I'm not sure if this is because we were converting from 1.3.0 and it used to be this way, but there wasn't really any clear documentation on how to set the properties from the Readme when I was trying to fix this.

I eventually found the correct usage from the tests

ansi_up/test/ansi_up-test.js

Line 585 in aaf041d

au.use_classes = true;

It'd be great to just have an example of setting properties right in the README.md

Can you send pull request?
@jennifer-shehane

Hi - please re-open if this is still an issue.