feat(migrate.js, bin/rethinkdb-migrate): Implement step option on up and down commands

[gristow]

See #20

• Updated docs
• Updated tests
• Fixed getExecutedMigrations to only return executed migrations (not all migrations)
• Replaced getAllMigrationsExecuted with getExecutedMigrations

[coveralls]

Coverage decreased (-0.08%) to 97.279% when pulling 8414e4c on gristow:issue-20/step-option into bd3cb1b on vinicius0026:master.

[gristow]

@vinicius0026 or @lirantal let me know if you want me to push commits to address codeclimate & coveralls failing tests. (I don't especially agree w/ codeclimate's suggested refactor but can get creative if need be...)

I think despite the 0.08% decrease in coverage the testing situation is better and more thorough now... but I can go back and catch that one LOC that became uncovered if you want. :-P

[vinicius0026]

Thank you for this PR, @gristow, very good work!

@lirantal do you want to go over it? I'll merge this, update dependencies and release v1.4 when you're good with this PR.

[lirantal]

I haven't thoroughly reviewed but this looks like a great addition!

[gristow]

Thanks for merging & publishing!