Leader key implementation (#326)

* implements leader key for planck experimental

* allows override of leader timeout

* adds ability to use the leader key in seq

* fixes leader keycode

* adds chording prototype

* fixes keycode detection

* moves music mode to quantum.c

* disables chording by default

* updates process_action functions to return bool

Merge branch 'master' of https://github.com/jackhumbert/qmk_firmware

Merge remote-tracking branch 'refs/remotes/jackhumbert/master' into pc_planck

Merge remote-tracking branch 'refs/remotes/jackhumbert/master'

Merge branch 'master' of https://github.com/jackhumbert/qmk_firmware

Merge pull request #182 from Vifon/modifier-release-fix

Fix the layer-dependent modifiers handling

Merge pull request #5 from jackhumbert/master

Sync with upstream

Merge branch 'master' of https://github.com/jackhumbert/qmk_firmware into modifier-release-fix

Merge branch 'modifier-release-fix' of https://github.com/eltang/qmk_firmware into modifier-release-fix

Update action.h

Merge branch 'master' of https://github.com/jackhumbert/tmk_keyboard

Merge remote-tracking branch 'jackhumbert/master' into make-keymap-names-consistent

Merge remote-tracking branch 'jackhumbert/master' into improve-buildguide

Merge remote-tracking branch 'origin/master'

Merge branch 'master' of https://github.com/jackhumbert/qmk_firmware

Merge pull request #214 from cdlm/action-hook

Add per-event user hook function

Rename function to be keyboard-specific

Add per-event user hook function to QMK

Cut the memory consumption of PREVENT_STUCK_MODIFIERS in half

Expose the pressed_actions_cache global variable