Add hosts addon

Requested in #86.

This allows for an easy way to add a hostname to the hosts file linking
to 127.0.0.1.

Usage:

Add the following to the .travis.yml file to link foobar.local to
127.0.0.1:

addons:
  hosts:
    - foobar.local
Add IPv6 support to hosts addon

Update ::1 as well as 127.0.0.1 for the added hostname to enable both
stacks for the hostname.
Insert appropriate amount of backslashes
Update sed command line options

The version of sed on our servers doesn't like "-i ''", it thinks that
"''" is another file name and then complains it can't find a file with a
blank filename.
Add a fold around the hosts addon
Merge pull request #101 from travis-ci/hh-hosts-addon

Add hosts addon
Closes #86