feat(plugins): add support for plugins

[chimurai]

Description

• Write plugins to extend and share proxy functionality, while keeping the core functionality small and clean.
• Exposes internal http-proxy instance; ie. to subscribe to its events directly.

Motivation and Context

• expose ProxyServer #190
• Expose HTTP Proxy middleware, so it can be used outside express #687

Allow plugins to be created for functionality like:

• pathRewrite not matching regex pattern with variables #538
• How to change cookies secure setting? #477
• secure: false does not remove cookies secure -flag #237

How has this been tested?

• unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage increased (+0.01%) to 98.799% when pulling a8f7fde on plugins into b674a71 on v3.