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.
Rather than limiting the user to two of Travis' original options, could we use:

```ruby
Thor::Options.to_switches(some_subset_of_our_options)
```

to pass any options given (beyond `--environtment` and `--path`) to `travis encrypt`? There are quite a few options we'd be missing out on otherwise.

The tricky part is the possibility of excluding documentation for those options in the help text for the Figaro command. Maybe it's possible to somehow require the Travis command and somehow include its option docs. I don't know. Thoughts?