Add option to prevent halt when NFC tag not found on begin()

[smartin015]

I'm working on a project where an NFC reader may not always be attached. The current implementation of begin() halts if the device isn't found on init - this is kind of a dealbreaker for my use case.

So, my change adds an optional flag to NfcAdapter::begin() that allows the user to bypass the halt behavior. The behavior without specifying this flag is unchanged, so there should be no worry about backwards compatibility.

I've also made begin() return true if the NFC board was initialized, and false otherwise to aid in potential sensing / retry logic on the user side.

Thanks for maintaining this great repo :)