WIP: enable to set granular vars for single build

Extend Travis::CommitCommand with the concept of assignments.
For example, include this in the commit message...

    # Escape the ci to avoid interpretation.
    [\c\i fail_fast = true]

...to get interpreted as `[{ 'fail_fast' => 'true' }]` and
exported to the build environment as `fail_fast=true`.

I'm not sure at this point how to delegate the key-value pairs
to the builder. /me needs to understand code better or get help.

Use case 1: Be able to set things like seeds for the test run
as mentioned at https://github.com/travis-ci/travis-core/issues/161

Use case 2: Be able to override default build configuration for
a single build. Example...

    RSpec.configure do |c|
      # Fail fast to reduce duration of test runs.
      # IOW get out of the way so that the next pull request gets tested.
      # Override for a single build using a Travis commit command.
      c.fail_fast = ENV['fail_fast'] == 'true'
    end