Add Leprechaun (#2206)

Leprechaun is tool where you can schedule your recurring tasks to be performed over and over. It supports cron, webhooks and classic job scheduling

**Please provide package links to:**

- github.com repo: [here](https://github.com/kilgaloon/leprechaun)
- godoc.org: [here](https://godoc.org/github.com/kilgaloon/leprechaun)
- goreportcard.com: [here](https://goreportcard.com/report/github.com/Kilgaloon/Leprechaun)
- coverage service link [![codecov](https://codecov.io/gh/Kilgaloon/Leprechaun/branch/master/graph/badge.svg)](https://codecov.io/gh/Kilgaloon/Leprechaun)

**Note**: that new categories can be added only when there are 3 packages or more.

**Make sure that you've checked the boxes below before you submit PR:**
- [x] I have added my package in alphabetical order.
- [x] I have an appropriate description with correct grammar.
- [x] I know that this package was not listed before.
- [x] I have added godoc link to the repo and to my pull request.
- [x] I have added coverage service link to the repo and to my pull request.
- [x] I have added goreportcard link to the repo and to my pull request.
- [x] I have read [Contribution guidelines](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#contribution-guidelines), [maintainers note](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#maintainers) and [Quality standard](https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md#quality-standard).

README.md

@@ -1481,6 +1481,7 @@ See [go-hardware](https://github.com/rakyll/go-hardware) for a comprehensive lis
 * [jsongo](https://github.com/ricardolonga/jsongo) - Fluent API to make it easier to create Json objects.
 * [jsonhal](https://github.com/RichardKnop/jsonhal) - Simple Go package to make custom structs marshal into HAL compatible JSON responses.
 * [kazaam](https://github.com/Qntfy/kazaam) - API for arbitrary transformation of JSON documents.
+* [leprechaun](https://github.com/kilgaloon/leprechaun) - Job scheduler that supports webhooks, crons and classic scheduling
 * [lrserver](https://github.com/jaschaephraim/lrserver) - LiveReload server for Go.
 * [mc](https://github.com/minio/mc) - Minio Client provides minimal tools to work with Amazon S3 compatible cloud storage and filesystems.
 * [mergo](https://github.com/imdario/mergo) - Helper to merge structs and maps in Golang. Useful for configuration default values, avoiding messy if-statements.