_get_package_name Update doc.
_get_package_name can get feature from branch name

We also add new function (_get_package_feature) as an alias of
the function (_get_package_name). By providing the feature from
the feature branch, we can quickly build new package name with
some special features without changing the PKGBUILD and/or
creating new duplicate branch
Combine three functions _get_*_from_tag to one

By supporting :feature in (_get_package_name_from_tag) we can
make this functions support all features of other two functions.
We do not need to maintain 3 different ruby regular expressions
Update functions documentation
_get_package_feature can get value from environment
s-makepkg: + notes for PACKAGE_FEATURE
Clean up some code
_get_package_feature MAY return "patch"

If we are working on the feature branch "p_foobar" here
"foobar" is the package name, it means we do not provide
any feature string in the branch name. We just return
"patch" in this case. Use can override this by providing
the environment variable PACKAGE_FEATURE though
s-makepkg will send PACKAGE_FEATURE to makepkg