Update Go (1.18) and dependencies

Merge pull request #10 from fastbill/go-update-1.18

Update Go and dependencies

Update to Go 1.15

Merge pull request #9 from fastbill/go-update

update linter, ci, deps and fix linter errors

v3.1.1: Merge pull request #8 from fastbill/hotfix/add-mailermock

Add template mailer mock

v3.1.0: Merge pull request #7 from fastbill/hotfix/PR6-followup

Tight up template mailer implementation

Introduce template mailer

v3.0.0

Bump package to v3

Upgrade to Go 1.13

Merge pull request #5 from fastbill/upgrade-go113

Upgrade to Go 1.13

Fix Goroutine leak

Merge pull request #3 from fastbill/fix-goroutine-leak

fix goroutine leak

Initial release

Merge pull request #1 from fastbill/switch-to-mit

Change LICENSE to MIT