Include index.d.ts in package (#647)

Typescript type definitions were excluded from the built package via
`.npmignore`. This commit adds an exception for the file so that the
type definitions are installed correctly.

Author: romandecker

.npmignore

@@ -12,3 +12,4 @@ __mocks__
 !LICENSE
 !README.md
 !package.json
+!index.d.ts