Added default value to CssWriter.write map option

[oscarmattsson]

A lot of the examples omit the map parameter for the function in the css option. For example css: css.write('bundle.css'). When using typescript in the rollup config, this displays an error because the map parameter is required.

The write function seems to have a condition for explicit false values of the map parameter and for all other values it runs the else statement. By setting the map default value to true, it should behave exactly as before.

Also updated the typings to reflect this change.

[Conduitry]

Do we want to swap around the condition inside that method so that it's just if (map) { ... } else { ... }? Yes that would change the behavior in certain situations (i.e., any non-undefined falsy value would now cause sourcemaps to not be generated), but I think the current behavior there is more confusing. (And what's supposed to happen when you explicitly pass a value other than false isn't described in the readme anyway.)

[benmccann]

that sounds like a reasonable suggestion to me