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 facebook/flow@f942d96?

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

[daniellizik]

@goto-bus-stop flow@0.96.0 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?