Allows the user to override $GO111MODULE environment variable through
ale options. This gives control over the default behavior of Go module
resolution.
Golang documentation:
https://github.com/golang/go/wiki/Modules#how-to-use-modules
Add `ale#Go#EnvString()` function to make it easy to add similar Go
environment variables in the future.
Use the new `EnvString` function in all available Go tools callbacks
& update tests
Also add test of linter command callback for `gofmt`
* Add support for https://github.com/saibing/bingo
* Add docs for ale-go-bingo
* Use go.mod when found
* Add test for bingo FindProjectRoot
* Simplify ale_linters#go#bingo#GetCommand
