add passphrase option

[chris--jones]

This PR adds the passphrase option for a PEM certification - fix #162
I had to copy and paste some code from the union library (trimmed as much as possible) to add a shim to enable configuring ssl passphrase

What changes did you make?
Added the option to read passphrase (from environment variable as recommended in previous conversations)
Added a shim to allow for the missing functionality from union
Updated README to reflect the new option, with an example of creating a cert

Provide some example code that this change will affect, if applicable:

  this.server = options.https.passphrase
    // if passphrase is set, shim must be used as union does not support
    ? require('./shims/https-server-shim')(serverOptions)
    : union.createServer(serverOptions);

Is there anything you'd like reviewers to focus on?
I realise this might be controversial, but given that union has not been updated in some time I think this is an ok short-term solution until a longer term one is implemented, either:

• fork union and make the fix there
• re-implement unions functionality within this project
• replace union with another library

Of all these options I think the best one is to find a replacement for union, but as a relative newcomer to this code-base I'm not sure specifically what functions this library is supporting nor am I brave enough (yet) to do any major refactoring.

Please ensure that your pull request fulfills these requirements:

• The pull request is being made against the master branch
• Tests for the changes have been added (for bug fixes / features)
• New features/options have been documented in:
  • http-server --help text (n/a)
  • README.md
  • doc/http-server.1

What is the purpose of this pull request? (bug fix, enhancement, new feature,...)
Enhancement / new feature - #162

[thornjad]

Tests are failing with this error:

test/main.test.js http-server main TypeError: Cannot read property 'passphrase' of undefined:
     Error: TypeError: Cannot read property 'passphrase' of undefined
      at test/main.test.js:283:9

[thornjad]

Awesome! Thank for for tackling a very old missing feature!