Add new backend: Vegeta

[s-radyuk]

Goal

Add tsenart/vegeta as a new backend for Kangal.

Acceptance Criteria:

vegeta is added to Kangal as a new load generator backend. Readme is updated accordingly. Kangal can create loadtests with vegeta type.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.