Fix bin/semver runtime error

[pjohnmeyer]

Changes since v5.5.1 have introduced an error when running the
semver CLI:

ReferenceError: options is not defined

This commit fixes this problem, then adds a few sanity tests
that run the CLI to hopefully ensure this doesn't happen again.

[coveralls]

Coverage remained the same at 97.454% when pulling b177fe1 on pjohnmeyer:fix-cli-error into 829858a on npm:master.

[pjohnmeyer]

I make this pull request because I was looking at working on #224 and possibly #146, but ran into this issue.

I think it might make sense to, in a future PR:

• Move the actual logic of the CLI into a different requirable file, like semver-cli.js, so it can be tested without using execSync.
• Change bin/semver to a wrapper that basically just does something like the below, but with additional error handling:

var argv = process.argv.slice(2);
var main = require('../semver-cli').entryPoint;
main(argv);