Add option implementing "always 2 spaces" indent rule

Add `clojure-default-indent-defun` customizable variable that, when
true, defaults indenting of all otherwise-unspecified function/macro
forms to defun-style (~ always 2 spaces).

This is a popular option in other environments, and IMO has some significant advantages:

1. Any codebase can be reliably formatted without special configuration to account for local macros.
2. Longer function (and Java method!) names can't adversely affect the width of a code body.

Of course, `clojure-default-indent-defun` is defaulted to `nil`, preserving current behaviour.