Add go.mod, fix import path for github.com/bouk/monkey

[ghetzel]

Hello! Recently, it appears that the github.com/bouk/monkey dependency has rehomed their module to bou.ke/monkey. For upstream modules that use the new(ish) Go modules feature, this import path is now causing build errors:

go get ./...
go: github.com/bouk/[email protected]: parsing go.mod:
	module declares its path as: bou.ke/monkey
	        but was required as: github.com/bouk/monkey

I have made the necessary fixes in my fork, and wanted to PR them back to you if you are interested.

1. Adds a go.mod and go.sum file to the project to track versions of dependencies.
2. Changes all instances of the import path from github.com/bouk/monkey to bouke/monkey.

Thank you for this package.

Cheers,

Gary