Add option implementing "always 2 spaces" indent rule

Add `clojure-defun-style-default-indent` customizable variable that, when
true, defaults indenting of all otherwise-unspecified function/macro
forms to defun-style (~ always 2 spaces).
documentation of baseline indentation impl + diffs when "always 2 spaces" is enabled
Merge pull request #181 from cemerick/master

Add option implementing "always 2 spaces" indent rule