added disableDynamicPayloads()

[soligen2010]

Ack payloads are also disabled at the same time because they depend on
dynamic payloads.

This is needed because I have an old protocol where the TX and RX use fixed length, and want the new code using dynamic lengths to be backwards compatible with the old receivers.

[soligen2010]

Since TMRh20 added me as a collaborator, I can now merge pull requests. Is there any process I should follow here? (seems like there should be a review or something first). My pull request is pretty simple and there is a lot in this library I don't fully understand. I just don't want to step on anything. Any info on the process you guys use for development/publishing/versions/reviews, etc on this library?

[Avamander]

Does enableDynamicPayloads restore the state as it was before disableDynamicPayloads?

[soligen2010]

disableDynamicPayloads turns off both dynamic payloads and ack payloads (because ack payloads needs dynamic payloads to be enabled). enableDynamicPayloads will only turn the dynamic payloads back on. If you were using ack payloads previously and want to re-enable then too, then these would have to be explicitly turned back on in code. This seems like reasonable behavior to me because there is a dependency here, and next time the dynamic payloads are enabled, there is no way to know if the user would want ack payloads re-enabled too or not. Seem right to you?

[Avamander]

Please make a commit that indents line 1263 as the rest of the code. Then I can squash and merge this, though I think the comment about disabling ACK should also include that it has to be manually re-enabled.