Replace 'run' command with 'compile' for travis.rb

Instead of the limited use-case 'run' command, which runs
given *stages* of a job, we introduce the 'compile' command,
which will display the bash script for the given job.

The usage is explained in README.
Handle 'env' and 'os' in no-arg case
Minor refactoring in Travis::CLI::Compile#run
Tweak error message for local compilation with 'os' key
Tweak CLI usage in README
Documentation fixes
Merge pull request #353 from travis-ci/ha-feature-cli-compile

Replace CLI plugin command `run` with `compile`