#7 Add support for start/end date (with patch)

[Kevin Bryan]

I find it useful to set up a vacation notification in advance, and to have it end without my intervention. I submit a patch (should apply against git 2ea2c574b5) that adds these features. It won't send auto responses until after the start time. After the expires time It will stop sending auto responses, and warn (via system log) users when they have left a vacation forward file in place.

-s 'YYYY-MM-DD hh:mm:ss' (starts)
-e 'YYYY-MM-DD hh:mm:ss' (ends)

Notes:
1) German man page entries done with Google Translate.
2) Only tested compiling with gcc on Linux
3) Patch may need whitespace changes for formatting
4) Hours/minutes/seconds are optional, so '2017-04-12' means midnight, '2017-04-12 12' means noon.