Support $GO111MODULE with Go tooling

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`

ale_linters/go/gometalinter.vim

@@ -14,11 +14,13 @@ function! ale_linters#go#gometalinter#GetCommand(buffer) abort
     " be calculated to absolute paths in the Handler
     if l:lint_package
         return ale#path#BufferCdString(a:buffer)
+        \   . ale#go#EnvString(a:buffer)
         \   . '%e'
         \   . (!empty(l:options) ? ' ' . l:options : '') . ' .'
     endif
 
     return ale#path#BufferCdString(a:buffer)
+    \   . ale#go#EnvString(a:buffer)
     \   . '%e'
     \   . ' --include=' . ale#Escape(ale#util#EscapePCRE(l:filename))
     \   . (!empty(l:options) ? ' ' . l:options : '') . ' .'