For the release candidates, allow passing the version (e.g. "3.1.1-rc")
to post-release.sh on the command line and document this.
Also don't commit automatically, this is annoying, especially as the
script doesn't check for errors.
Finally, fix the problem with the CHM file name: it must be zipped,
presumably to avoid problems with some firewalls blocking downloading
CHM files (as there is really no advantage in compressing the already
compressed CHM file otherwise).
Don't assume they're run from the directory containing them, especially
as this is not how they are shown to be invoked in the release
instructions.
Now running ./build/tools/pre-release.sh works too.
Update dates in various files before the release automatically.
Also automatically update SHA-1 of the release files both before the
release (to zero them) and afterwards (to use the correct values).
Closes https://github.com/wxWidgets/wxWidgets/pull/443
