Enforce case-sensitivity on all regexps

[loverobean]

As a long-term follow up to Issue#61, it would be best to go through all regexps in this mode and set case-fold-search nil, or perhaps to define a version of looking-at which will always be case-sensitive regardless of the value of case-fold-search.

(In this particular case, the behavior with case-folding is a little more convenient, because if a user attempts to use a lowercase letter as a flag, he can correct his mistake using beancount-transaction-clear or beancount-transaction-flag, which would not be possible if the regexp match failed.

That said, in general, the code would be more precise if every regexp match were case-sensitive.

(defun beancount-transaction-clear (&optional arg)
  "Clear transaction at point. With a prefix argument set the
transaction as pending."
  (interactive "P")
  (save-excursion
    (save-match-data
      (let ((case-fold-search nil))
      (let ((flag (if arg "!" "*")))
        (beancount-goto-transaction-begin)
        (if (looking-at beancount-transaction-regexp)
            (replace-match flag t t nil 2)))))))