Support for using speedbump as a library

[kffl]

As of now, speedbump can only be used as a standalone program. There are some potential use cases for calling speedbump as an external library from within other Go code (i.e. for the purpose of adding programmatic TCP delay during a e2e test or a load test).

In order for speedbump to be used as a library, the following changes need to be made:

• adding support for graceful shutdown (i.e. .Stop() method on a Sppedbump instance), related to Graceful shutdown #6
• adding required go docs comments to exported members
• exporting some currently unexported config struct fields