Add option to customize the timestamp format

Add a new -d switch with which to customize the format of the timestamp inserted
into the commit message.
This prepares commit message customization as mentioned in #8.
Change option letter for push, expand help message

Change the option letter for auto-pushing after a commit from -p (for 'push') to
-r (for 'remote') since I think that makes a bit more sense.
Expand the help on auto-pushing.
Change construction method for commit message

Instead of having a fixed structure of PREPEND+DATE+APPEND for the commit
message, we now use a single commit message template string, in which the
placeholder %d will be replaced by the formatted date and time (if any).
This is further preparation for implementing a customizable commit message
as requested in #8.
Add option to customize commit message

The commit message used by gitwatch can now be customized without having to
touch the code of the script. The new -m switch can be used for this, followed
by the commit message "template"; upon commit, all occurences of %d in that
templace string will be replaced by the formatted date/time (see also the -d
switch).
Closes #8.