Fix type error and add typedoc build step.

[Bubblyworld]

There's currently a type error in map.ts as none of the build steps do type
checking at the moment:

Error: node_modules/@jspm/import-map/src/map.ts:40:5 - error TS2322: Type 'URL | { map?: IImportMap; mapUrl?: string | URL; rootUrl?: string | URL; }' is not assignable to type 'URL'.
  Type '{ map?: IImportMap; mapUrl?: string | URL; rootUrl?: string | URL; }' is missing the following properties from type 'URL': hash, host, hostname, href, and 9 more.

40     this.mapUrl = mapUrl;
       ~~~~~~~~~~~

I've added a typedocs build step which does checking, so we should be able to
build docs upstream for the jspm generator and CLI now.

[CLAassistant]

All committers have signed the CLA.

[Bubblyworld]

Typedocs are pretty!

[guybedford]

Great to see, thank you!