Allow head-hint to be dynamic

Allow head-hint to be dynamic

Instead of only a string like before, the head-hint can be anything that
evaluates to a string.

Example:

(defhydra hydra-test (:columns 2)
  "Test"
  ("j" next-line (format-time-string "%H:%M:%S" (current-time)))
  ("k" previous-line (format-time-string "%H:%M:%S" (current-time)))
  ("h" backward-char (format-time-string "%H:%M:%S" (current-time)))
  ("l" forward-char (format-time-string "%H:%M:%S" (current-time))))

Pressing "hjkl" will refresh the hint, and thus update the current time.
Note that the hint needs to evaluate to a string at both compile-time
and run-time. The column formatting depends on the compile-time result.

Fixes #160