warn/throw on mismatched engine/platform all the time, not only for newly added modules

[isaacs]

See npm/rfcs#195

• Currently we call checkEngine()/checkPlatform() in lib/arborist/reify.js, but only for nodes being added to the tree in reifyNode().
• Either move that check to loadActual and loadVirtual, so that we can warn when we're initially building the tree, or put it in buildIdealTree since we only care about it when we're going to be modifying stuff? (Ie, should npm ls warn about it? Should npm ls --engines-strict fail entirely and not even show a tree?)
• Since it's probably too aggressive to fail entirely if a package in the virtual/actual tree has an engine/platform mismatch, we can collect these errors up to the top level, and then have buildIdealTree() decide to throw if there are any engine mismatches and options.enginesStrict is true.

[isaacs]

Also: we should track the node/npm version in the package lock, and only run this check (at least in loadVirtual) if it doesn't match the current node/npm versions.

[markablov]

@isaacs should i implement that by my own? arborist looks not very familiar to me, but i think i can handle that.

[bonkydog]

Hi, @markablov. Thanks, but I just started working on this yesterday.

[darcyclarke]

Should land in the next beta release. Work landed in arborist here: #138