emacs-mode: Prefix every function/variable/macro with ledger-.

This is the common convention in Emacs Lisp. Not following it might
cause problems with other packages as functions/variables/macros are
always visible to other modules.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
ledger-mode: Add `ledger-remove-effective-date'.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Improve `ledger-insert-effective-date'.

The function now replaces already existing effective dates in the same
line.

With a prefix (C-u C-c C-t), remove the current effective date.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>