handle edge-cases when setting alias keys

[bcoe]

In the ongoing effort to make yargs unsinkable, this pull request handles edge-cases when setting aliases. This behavior used to fail:

var argv = require('yargs')
  .alias('x', 'y')
  .alias('y', 'z')

As did this:

var argv = require('yargs')
  .alias('z', 'zm')
  .alias('zoom', 'zoop')
  .alias('zm', 'zoom')

We're now clever about merging together alias groups as aliases are defined, solving these transitive edge-cases ... definitely an edge case, but I'm a bit OCD about making yargs handle every edge-case I think of!

reviewers: @nexdrew

[nexdrew]

s/the/they/

[nexdrew]

@bcoe You're a bit crazy, but I like your style! 👍

[nexdrew]

LGTM