defaultBlockTag type can accept null

[dongliang1993]

defaultBlockTag can accept null in doc, but types maybe wrong

now:

draft-js-utils/packages/draft-js-export-html/typings/index.d.ts

Line 16 in 4007f3e

defaultBlockTag?: string;

should be:
defaultBlockTag?: string | null;

[pereznieto]

My team was also being affected by this issue, so I've raised a PR to add the suggested change: #201