README: Updating the configuration section

Replaced the two-method reconfiguration with the new single-method
approach.
config: Adding an option to disable configuration

Now you can do

```
Tco::configure do |c|
  c.options["disabled"] = true
end
```

to disable colouring through the whole app.

Related to #5.
Removing trailing whitespace
spec: Adding uncommited tests

I forgot to commit those ...