Custom style from shields.io

[uniibu]

This is an updated version of the previous PR #8 without the excessive code refactoring and only using greenkeeper's default color schemes.

Status:

• Travis-ci Build

Major Change:

• Added support to shields.io's style parameters and this should also satisfy greenkeeperio/greenkeeper#416 and #16. With this PR, greenkeeper can now accept a style paramater and supports flat flat-square and plastic style from shields.io.

Minor Changes:

• Added example badges on README
• Added tests for the new style query

Sample query: https://badges.greenkeeper.io/owner/repo.svg?style=plastic

Note: Shields.io's for-the-badge style is not included because the current npm-published gh-badges@1.3.0 does not support the said style.