This adds an action, `ACTION_SWAP_HANDS`, that swaps the the keys on the keyboard across a keymap-defined hemisphere in order to support one-hand typing without requiring a separate one-handed layer. See updated `doc/keymap.md` for more information.
This commit is mostly a cherry-pick from `ahtn` at
https://github.com/tmk/tmk_keyboard/pull/255.
These are the changes:
* Adds ACTION_LAYER_ONESHOT
* Adds ONESHOT_TAP_TOGGLE
* Mentions sticky keys in the docs on oneshot.
