Add .npmignore to make the published module smaller and work around a bug in Flow

[dumbmatter]

This will make everyone's life slightly better by making the module distributed through npm a bit smaller.

And it will save users of Flow some grief, because of facebook/flow#5898.

[anfedorov]

looks good & would have appreciated not having to manually fix the flow error. eta?

[goto-bus-stop]

Is this fixed in flow since https://github.com/facebook/flow/commit/f942d9676d174d3469ed532770934063518ce4f5?

Shipping tests and examples with packages is handy if users are working offline or if they need to debug something.

[daniellizik]

@goto-bus-stop [email protected] is still identifying this as an error for me

The main problem is, I have no idea why putting

[ignore]
node_modules

in my .flowconfig still reads all package.json files in node_modules...have you found another workaround?