Add travis:encrypt command

I took a pass at implementing the feature discussed in #8.

I exposed the `--add` and `--override` options to allow the end user to control edits to .travis.yml.
Or we could pick a larger subset (maybe all) of the Travis options and manually document them. Then the `travis_options` method might look more like:

```ruby
Thor::Options.to_switches(options.except(:environment, :path))
```

Of course, `Hash#except` is a Rails thing but you get the idea.