Command to run flow type checker

[Gozala]

I see there is #213 which got stalled. @vmx also mentioned ipld/js-ipld-stack#6 (comment) he'd love inline types through flow comment syntax, which would require less changes to the established workflow than previous attempt to introduce some types.

I would like to help make this happen & I suspect https://github.com/multiformats/js-cid might be a good place to start, however I suspect there will be a desire to crease something like aegir type-check to take care of it all.

I would like to start a discussion here so we can workout the best strategy to go about this.