Implement version --outdated.

[rishson]

Fixes #67, #68, addresses some of #73. Adding this behaviour has meant moving the existing version functionality to a Command because the --outdated check is async. This has caused a limited refactor. Where possible, I have not altered the code layout or functionality to keep this PR small, however, I have done the following:

• fixed all tslint errors.
• increased the coverage as far as I can

Getting code coverage from 99.87% to 100% will require introducing extra dependencies or re-writing how the cli main entry func works, so I have not done this.

The functionality is limited to using npm as the cli module repository. The command interface now has group and name as optional properties, because these are specified by the version in-built command, rather than being derived from the command's enclosing directory name.

[jsf-clabot]

All committers have signed the CLA.

[tomdye]

Initial comments raised, still needs a more thorough review.

[codecov-io]

Current coverage is 99.10% (diff: 98.57%)

Merging #75 into master will decrease coverage by 0.27%

@@             master        #75   diff @@
==========================================
  Files            10         10          
  Lines           161        223    +62   
  Methods           0          0          
  Messages          0          0          
  Branches         14         19     +5   
==========================================
+ Hits            160        221    +61   
+ Misses            1          0     -1   
- Partials          0          2     +2   

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[rishson]

Don't get the codecov. grunt test gives coverage of 99.57%, but codecov on ci says 97.87%. Even the summary directly above this comment shows that the PR raises line and branch coverage.

[agubler]

Nice @rishson, merge away at your own convenience 👍