Implement mark_old_as_done function.

If ~/.memorc file has a property MARK_AS_DONE with a valid day
(yyyy-mm-dd) value, notes older than that date will be automatically
marked as done.

Signed-off-by: Niko Rosvall <niko@ideabyte.net>
Make Memo a bit more Windows friendly.

On Windows files that start with a dot cause problems sometimes.
Now, on Windows, ~/.memorc is ~/_memorc and .memo is _memo.

Signed-off-by: Niko Rosvall <niko@ideabyte.net>
Update man page about new features.

Signed-off-by: Niko Rosvall <niko@ideabyte.net>
README should not say that Memo supports Windows native.

Memo compiles and works natively on Windows, but at the moment Memo is
not officially supported on Windows. Mostly because I don't use Windows
and I don't have access to a Windows machine. The situation might change
if someone wants to start maintaining Windows version of Memo.

Signed-off-by: Niko Rosvall <niko@ideabyte.net>