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`
In Go you can "vendor" packages by putting them in the `vendor/`
directory for a project. Adding the `-srcdir` argument makes `goimports`
pick up these packages, in addition to what you have in GOPATH.
Without this, `goimports` is not very useful, since most projects vendor
their packages.
