Provide a separate global mode

It should be possible to enable/disable smartscan-mode on a
per-buffer basis, so a global mode alone is not an ideal
solution.

This commit makes smartscan-mode a regular minor mode, and adds
a globalized version called global-smartscan-mode which will
enable it everywhere by default.

This commit also fixes a typo in the "Install" comments
(smartscan vs smartscan-mode), and re-formats the header so that
all information will be extracted from the Commentary by
package.el and made available by package repositories for when a
user presses "?" in the `list-packages` buffer.