Feature/pjh/classification extension

[pjhartzell]

Related Issue(s): Closes #22

Description: Adds the Classification extension to the QA Bands of new format Items. Specifically, adds classification:bitfields to the new format Item QA Band Assets. Not considered a breaking change, as the "legacy_l8" Item format is unchanged. Calling create_stac_item without changing the default values will produce a "legacy_l8" Item.

The Classification extension is not added to the new format Collections because the bitfields are not the same between sensors, and therefore can not be cleanly represented in item_assets. Also choosing not to add classification:bitfields to Collection summaries, as their context is not clear without being attached to a particular QA band.

PR checklist:

• [x] Code is formatted (run scripts/format).
• [x] Code lints properly (run scripts/lint).
• [x] Tests pass (run scripts/test).
• [x] Documentation has been updated to reflect changes, if applicable.
• [x] Changes are added to the CHANGELOG.
• [x] Examples are added to examples/

[gadomski]

In general we'll want to keep the version bump PR separate from the feature PR, but in this case its ok I reckon.

[pjhartzell]

Nah, let's do it right. I'll push an update.