Add manual bid support

[tfe]

⚠️ This does include a breaking API change. ⚠️

Generally adds ability to pass bid_amount parameter when creating ad sets.

But also changes ad set is_autobid parameter default to nil (was true). This is a breaking change for current users of this library, but I think it's essential for moving forward. Rationale:

• The previous behavior (defaulting to true) is not aligned with how the official FB SDK works, which is not opinionated about default parameter values. We shouldn't be opinionated this way either.

• In the future we want to support the bid_amount parameter. However with is_autobid defaulting to true, if someone were to pass bid_amount without explicitly turning off autobid, the request would fail. That seems like an obvious developer trap that we can avoid by making users be explicit about autobid when they want it.

Normally a breaking change like this would require a major version number increment, but since we're on major version 0 the public API is not expected to be stable, so I plan to just bump the minor version number.

[coveralls]

Coverage remained the same at 73.946% when pulling e3c32167a267b67c633850a802f55729c6946222 on add-manual-bid-support into 74057d52b4c8535d61f2b5fc8b6435753ab878c4 on master.

[coveralls]

Coverage decreased (-0.2%) to 73.755% when pulling 1431a0965c3f136833c462b75f51e37d97ab783c on add-manual-bid-support into 74057d52b4c8535d61f2b5fc8b6435753ab878c4 on master.

[coveralls]

Coverage remained the same at 73.946% when pulling fe5bc5cac284d3e0066f0c43d239b2fab83eca7b on add-manual-bid-support into 74057d52b4c8535d61f2b5fc8b6435753ab878c4 on master.