This commit adds (optional) support for additional sha256 checksum utilities
for newer versions of node.js and io.js that use sha256 checksums rather than
sha1.
If nothing is found to do a sha256 checksum on the client machine, a warning is
printed and things continue on as normal.
Following comments from @ljharb on incorporating some of @DomT4's PR
creationix/nvm#664, and making this checksum optional. If I could I would
gladly include this as an addon to the now closed PR creationix/nvm#664. I am
choosing not to file it onto that PR because it's closed and (currently)
significantly behind the master branch.
@DomT4 did the hard work of actually finding all the different ways in which
one could verify a sha256 checksum, I've just included those here in an effort
to move forward with sha256 checksum support.
Brandon Wood