Make NFCPushOptions optional for pushMessage

[alexshalamov]

In my opinion, by making NFCPushOptions optional, we can improve API usability and developers would need to type less code for main use-cases.

pushMessage(message);

// Instead of

pushMessage(message, {target: "tag", timeout: Infinity});

[zolkis]

If we can have sensible defaults, why not.
I expect that most of the pushes will be done to peers, and tags are much more often read than written.
If we need to set default values, then I am inclined to make "peer" the default target for a push.

Timeout could be set to Infinity as a default.

[zolkis]

We could also separate push (to peers) from write (to tags).
Then we could say

nfc.push(message1);
nfc.write(message2);

[zolkis]

Fixed by #88.