Replace Term::ANSIColor with a lighter solution

The `term-ansicolor` gem includes some extensive dependencies by way of
`tins`. This seems unnecessary given the extent of the use in `SSHKit`.
This commit replaces the dependency with the lighter `colorize` gem and
moves any color implementation behind the `SSHKit::Color` module, with a
matching interface.
Merge pull request #132 from onthebeach/feature/colorize

Replace Term::ANSIColor with a lighter solution