comment block for nav object mixin
improve corner cut mixin, add margin options. fixes #4, #16

- introduce margin override to mixin

- set mixin size default to base spacing * 2

- set margin default to 0
corner cut spacing

- corner-cut mixin should only generate default content when both overrides are false

- page headers need margin-override

- increase bottom margin on posts
post titles should be linked to the post. duh. fixes #27
comment colors, typography, spacing. fixes #26
exai: remove sticky footer

https://github.com/montchr/exai/commit/479834372d2888e1c99eb818d2b4eb6b7b2254a8
sticky footer overrides are no longer necessary without the sticky footer