export interface not supported in Flow

[coaxial]

Flow supports export interface, but importjs doesn't pick up on these statements when auto-importing.

A workaround is to use the same workaround as for older Flow versions:

interface iMyInterface { ... }
export type IMyInterface = iMyInterface

It would be nice if import-js supported the shorter export interface... syntax since Flow now supports it.

cf. facebook/flow#1131

[lencioni]

@coaxial I'd be happy to review a pull request that adds this feature and tests. I think you'd probably need to modify something around this area:

import-js/lib/findExports.js

Lines 94 to 108 in 6e88abf

	function findFlowTypeExports(node) {
	if (node.type !== 'ExportNamedDeclaration') {
	return [];
	}
	if (!node.declaration) {
	return [];
	}
	if (node.declaration.type === 'TypeAlias') {
	return [node.declaration.id.name];
	}
	return [];
	}