Implement a "truncate" flag to omit trailing zeros

If enabled, release.py will search for tags matching "v*", rather than
"v*.*.*", and will not create tags with trailing ".0"s. For example,
"1.0.1" truncated is still "1.0.1", but "1.1.0" is "1.1" and "1.0.0" is
just "1".
Update documentation for -t flag
Rework parsing to support truncation